Vanadium, Niobium, and Tantalum

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VOLUME VI. PART III. In ELBVBN VOLUMES. Medium 8vo. Cloth. Prices are net. Postage extra. A TEXT-BOOK OF INORGANIC CHEMISTRY. EDITED BT

J. NEWTON FEIBND, D.Sc., PH.D., F.I.C., Carnegie Gold Medallist.

An Introduction to Modern Inorganic Chemistry. By J. NEWTON H. F. V. B.So. FRIEND, D.Sc. (B'ham), Ph.D. (Wurz) ; LITTLE, to Ltd. W. E. S. VOLUME I. (Lond.), A.R.C.S., Chief Chemist Thorium, ; H. V. A. TURNER, D.Sc. (Lond.). The Inert Grases, By BRISCOE, D.Sc. (Lond.), A.R.C.S. A. JAMIESON WALKER, f" The Alkali Metals and their Congeners. By VOLUME II. B.A. F.I.C. i-xxvi +379. 20s. { Ph.D. (Heid.), (Q.U.B.), Pp. I. The Alkaline Earth Metals. By MAY SYBIL BURR (n&e 346. 20s. LESLIE), D.Sc. (Leeds). Pp. i-xxvi + JOSHUA 0. VOLUME III. PART II. Beryllium and its Congeners. By GREGORY, SYBIL D.Sc. B.Sc. (Lond.), F.I.C., and MAY BURR, (Leeds). (PARTPp. i-xxvi + 320. 18s. the Bare Earth Metals. ( Aluminium and its Congeners, including B.Sc. A.R.C.S., Chief Chemist to VOLUME IV. \ By H. F. V. LITTLE, (Lond.), 485. | Thorium, Ltd. Second Petition. Pp. xxviii + 18s. Carbon and its Allies. By B, M. CAVEN, D.Sc. (Lond.), F.LO. V, / VOLUME Edition. i-xxi +468. 18s. I Second Pp. PART I. Nitrogen. By E. B. R. PRIDEAUX, M.A., D.So., F.I.C., and H. LAMBOURNE, M.A., M.Sc., F.I.C. Pp.i-xxviii+242. 18s. PART II. Phosphorus and its Congeners. By E. B. R. PRIDEAUX, VOLUME VI. M.A., D.Sc., F.I.O., B. D. SHAW, B.Sc., Ph.D., and W. E. THORNEYCROFT, B.Sc. In Preparation. PART III. Vanadium, Niobium, and Tantalum. By SYDNEY MARKS, M.Sc., A.I.C. Pp. i-xxvi +222. TART I. Oxygen. By J. NEWTON FRIEND, D.Sc., and DOUGLAS F. Twiss, D.Sc., F.LC. Pp. i-xxvi +370, 18a. PART II. Sulphur, Selenium, and Tellurium. By REECE H. VALLANOE, M.Sc., DOUGLAS F. Twiss, D.Sc., and Miss A. R. VOLUME VII. RUSSELL, B.Sc. In Preparation. PABT III. Chromium and its Congeners. ByREEOEH. VALLANOB, M.Sc., A.I.C., and ARTHUR A. ELDRIDGE, B.Sc., F.LC. Pp. i-xxvi +380. 18s. Halogens and their Allies. By GEOFFREY MARTIN, D.Sc., VOLUME VIII. /The 1 Ph.D., and ERNEST A. DANOASTER, B.Sc. (Lond.). I. Cobalt, Nickel, and the Elements of the Platinum Group. By J. NEWTON FRIEND, D.Sc. (B'ham). Second Edition. VOLUME IX. Pp. i-xxvi +367. 18s. PART II. Iron and its Compounds. By J. NEWTON FRIEND, {PARTD.Sc. Pp. i-xxvi + 265, 18s. Metal-Ammines, with a General Introduction to the Theory VOLUME X. of Complex Inorganic Substances. By Miss M. M. J. SUTHER- {TheLAND, D.Sc., F.I.C. Pp. i-xxvi +260. 18s. Organometallic Compounds. PART I. Derivatives of the Elements of Groups I to IV. By ARCHIBALD E. GODDARD, M.Sc,, A.I.C., and DOROTHY GODDARD, M.Sc. Pp. i-xxviii +418. 25s. VOLUME XI PART II. Derivatives of Arsenic. By ARCHIBALD E. GODDARD,

M.Sc. s A.I.C. In Preparation. PART III. Derivatives of the Elements of Groups V to Vm (excluding Arsenic). By ARCHIBALD E. GODDARD, M.Sc., A.I.O. In Preparation.

LONDON: CHARLES GRIFFIN

Text-book of Inorganic Chemistry, 7ol. FJ., Part HL] [Frontispiece.

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VOLUME VI. PAET III YANADIUM, NIOBIUM, AND TANTALUM.

SYDNEY MARKS, M.So., A.I.C.

OF r : CMMEGIE ItiSiii'Jit ui lia

LIBRARY

LONDON: CHARLES GRIFFIN & COMPANY, LIMITED, 42 DBUEY LANE, W.0.2. 1929.

DURING the past few years the civilised world has begun to realise the advantages accruing to scientific research, with the result that an ever- increasing amount of time and thought is being devoted to various branches of science. No study has progressed more rapidly than chemistry. This science be may divided roughly into several branches : namely. Organic, Physical, Inorganic, and Analytical Chemistry. It is impossible to write any single text-book which shall contain within its two covers a thorough treatment of any one of these branches, owing to the vast amount of information that has been accumulated. The need is rather for a series of text-books dealing more or less comprehensively with each branch of chemistry. This has already been attempted by enterprising firms, so far as physical and analytical chemistry are concerned; and the present series is designed to meet the needs of chemists. inorganic One great advantage , of this procedure lies in the fact that our knowledge of the different sections of science does not progress at the same rate. Consequently, as soon as any particular part advances out of proportion to others, the volume dealing with that section may be easily revised or rewritten as occasion requires. Some method of classifying the elements for treatment in this way is clearly essential, and we have adopted the Periodic Classification with slight alterations, devoting a whole volume to the consideration of the elements in each vertical column, as will be evident from a glance at the scheme in the Frontispiece. In the first volume, in addition to a detailed account of the elements of Group 0, the general principles of Inorganic Chemistry are discussed. Particular pains have been taken in the selection of material for this volume, and an attempt has been made to present to the reader a clear account of the principles upon which our knowledge of modern Inorganic Chemistry is based. At the outset it may be well to explain that it was not intended to write a complete text-book of Physical Chemistry. Numerous excellent works have already been devoted to this subject, and a volume on such lines would scarcely serve as a suitable introduction to this series. Whilst Physical Chemistry deals with the general principles applied to all branches of theoretical chemistry, our aim has been to emphasise their application to Inorganic Chemistry, with which branch of the subject this series of text-books is exclusively concerned. To this end practically all the illustrations to the laws and principles discussed in Volume I. deal with inorganic substances. Again, there are many subjects, such as the methods employed in the accurate determination of atomic weights, which are not generally regarded as forming part of Physical Chemistry. Yet these are vii viii VANADIUM, NIOBIUM, AND TANTALUM. of subjects of supreme importance to the student Inorganic Chemistry and are accordingly included in the Introduction. Hydrogen and the ammonium salts are dealt with in Volume II., of the rare earth along with the elements of Group I. The position metals in the Periodic Classification has for many years been a source included in Volume with of difficulty. They have all been IV., along to be the most suitable the Elements of Group III., as this was found place for them. have an claim to be considered Many alloys and compounds equal in two or more volumes of this series, but this would entail unnecessary of and tin be dealt duplication. For example, alloys copper might certain with in Volumes II. and V. respectively. Similarly, double salts such, for example, as ferrous ammonium sulphate might very II. under and in Volume IX. logically be included in Volume ammonium, been overcome under iron. As a general rule this difficulty has by two or more metals or treating complex substances, containing bases, to the in that volume dealing with the metal or base which belongs For the of highest group of the Periodic Table. example, alloys copper and tin are detailed in Volume V. along with tin, since copper occurs ferrous ammonium earlier, namely, in Volume II. Similarly, sulphate in is discussed in Volume IX. under iron, and not under ammonium in IX. Volume II. The ferro-cyanides are likewise dealt with Volume But even with this arrangement it has not always been found easy For in the to adopt a perfectly logical line of treatment. example, chromates and permanganates the chromium and manganese function are to and chlorine as part of the acid radicles and analogous sulphur so that should be treated in the in sulphates and perchlorates ; they in the case volume dealing with the metal acting as base, namely, in Volume II. But the of potassium permanganate, under potassium with one another alkali permanganates possess such close analogies seems desirable. that separate treatment of these salts hardly They are therefore considered in Volume VIII. it is Numerous other little irregularities of a like nature occur, but indexes and cross- hoped that, by means of carefully compiled frequent to the texts of the volumes, the student will referencing separate ^ the information he experience no difficulty in finding requires. Particular care has been taken with the sections dealing with the are not atomic weights of the elements in question. The figures given been necessarily those to be found in the original memoirs, but have the recalculated, except where otherwise stated, using following fundamental values : Hydrogen = 1-00762, Oxygen = 16-000. Sodium = 22-996, Sulphur = 32-065. Potassium = 39-100. Fluorine = 19-015. Silver = 107-880. Chlorine = 35-457. Carbon = 12-003. Bromine =* 79-916. Nitrogen = 14-008. Iodine = 126-920. By adopting this method it is easy to compare directly the results of earlier investigators with those of more recent date, and moreover it renders the data for the different elements strictly comparable throughout the whole series. Our aim has not been to make the volumes absolutely exhaustive, GENERAL INTRODUCTION TO THE SERIES. ix as this would render them unnecessarily bulky and expensive ; rather has it been to contribute concise and accounts of the various suggestive % topics, and to append numerous references to the leading works and memoirs dealing with the same. Every effort has been made to render these references accurate and reliable, and it is hoped that they will prove a useful feature of the series. The more ijnportant abbreviations, which are substantially the same as those adopted by the Chemical Society, are detailed in the subjoined lists, pp. xv-xvii. The addition of the Table of Dates of Issue of Journals (pp. xix-xxvi) will, it is hoped, enhance the value of this series. It is believed that the list is perfectly correct, as all the figures have been checked against the volumes on the shelves of the library of the Chemical Society by Mr. F. W. Clifford and his staff. To these gentlemen the Editor and the Authors desire to express their deep indebtedness. In order that the series shall attain the maximum utility, it is necessary to arrange for a certain amount of uniformity throughout, and this involves the suppression of the personality of the individual author to a corresponding extent for the sake of the common welfare." It is at once my duty and my pleasure to express my sincere appre- ciation of the kind and ready manner in which the authors have accommodated themselves to this task, which, without their hearty co-operation, could never have been successful. Finally, I wish to acknowledge the unfailing courtesy of the publishers, Messrs. Charles Griffin & Co., who have done everything in their power to render the work straightforward and easy. J, NEWTON FRIEND. July 1929

PREFACE.

THIS volume deals with the chemistry of vanadium, niobium, and tantalum. These elements are all of comparatively recent discovery, none of them being known before the nineteenth century. Vanadium, niobium, and tantalum are metals which possess the capacity of forming both basic and acidic oxides, and, as they also display several degrees of combining power, the types of compounds they produce are very varied and often complex. The chemistry of these elements is, therefore, of considerable interest. In consequence of the difficulty experienced in separating niobium and tantalum satisfactorily, the compounds of these two elements have not been extensively their fiirther offers explored ; investigation certainly an attractive field for research. Vanadium has found important application in the manufacture of special steels, and tantalum is being increasingly employed for electrolytic "rectifiers." It seems probable that the industrial application of these elements will increase in the future, and that uses will also be found for niobium. In this volume only the inorganic compounds of the three elements are described, but references to the literature of the better known organic compounds have been given. The literature references throughout are complete up to December 1928, and every effort has been made to render them accurate. Information on several points has been obtained from Abegg & Auerbach's and from Gmelin-Kraut's Handbuch der anorganischen Chemie. The Author's best thanks are due to the Editor, Dr. J. Newton Friend, for his advice and for reading both the manuscript and the proofs, as well as to Miss A. R. Russell, B.Sc., A.I.C., for reading the final proofs. SYDNEY MARKS.

July 1929.

VOL. vi. : in.

CONTENTS. PAGE

. . iv THE PERIODIC TABLE (Frontispiece) .

GENERAL INTRODUCTION TO THE SERIES . . vii PREFACE ...... xi

LIST or CHIEF ABBREVIATIONS . . . . . xv

TABLE OF DATES OF ISSUE OF JOURNALS . . . xix CHAPTER L General Characteristics of the Elements of Group V., Subdivision A .... 3 The Vanadium Group of Elements Relation to Elements of Subdivision B Similarities to Chromium Comparative Study of Vanadium, Niobium, and Tantalum Typical Compounds.

CHAPTER II. Vanadium and its Alloys . . .9 Occurrence of Vanadium Minerals containing Vanadium History of the Metal Commercial Sources Extraction and Preparation Physical Pro- perties, Spectra of Metal Physiological ActionChemical Properties- Atomic Weight Uses of Metal Alloys.

CHAPTER III. Compounds of Vanadium . . .30 of Oxides General Properties of Vanadium Compounds Heats of Formation Electromotive Behaviour of Metal Vanadium and Hydrogen Vanadium and Fluorine Double Halides and their Constitution Vanadium and Chlorine Vanadium and Bromine Vanadium and Iodine Oxides Vanadates: Hypovanadites or Vanadites Vanado-vanadates General, Orthovanadates, Stannovanadates or Vanadostannates, Pyrovanadates, Metavanadates, Polyvanada,tes or Acid Vanadates, Double Vanadates Heteropoly-acids containing Vanadium : General, Vanado-phosphates, Molybdo-vanadophosphates, Tungsto-vanadophosphates, Vanado-arsenatea, Molybdo-vanadoarsenates, Tungsto-vanadoarsenates, Molybdo-vanadates, Tungsto-vanadates, Uranyl-vanadates, Molybdo-vanadosilicates, Tungsto- vanadosilicates, Vanado-selenites, Vanado-tellurites, Vanado-iodates, the Pervana- Vanado-periodates, Oxalo-vanadates Pervanadic Acid and datesVanadium and Sulphur, Vanadium Alums Vanadium and Selenium Vanadium and Chromium Vanadium and Nitrogen, Phosphorus, Arsenic Vanadium and Carbon, Vanadium and Cyanogen Vanadium and Silicon Vanadium and Boron.

CHAPTER IV. The Detection and Estimation of Vanadium . 109

: Volumetric Electro- Detection : Wet Tests, Dry Tests Estimation Methods, metric Methods, Colorimetric Methods, Electrolytic Method, Estimation by Measurements on Arc Spectrum, Gravimetric Methods.

CHAPTER V. Niobium and Tantalum . . .117 Occurrence Analyses of Niobites or Columbites, Tantalites, Pyrochlore, Extrac- Yttrotantalite, Fergusonite, Samarskite History of the Metals Estimation of tion of the Metals, Separation of Niobium and Tantalum xiii xiv VANADIUM, NIOBIUM, AND TANTALUM. PAGE Niobium and Tantalum: Gravimetric Methods, Volumetric Methods, Colorimetric Methods Detection of Niobium and Tantalum : Wet He- actions, Dry Reactions.

CHAPTER VI. Niobium and its Alloys . . . 134 Preparation of Metallic Niobium Physical Properties of the Metal, Optical Properties, Arc Spectrum Chemical Properties Electromotive Behaviour Atomic Weight Alloys.

CHAPTER VII. Compounds of Niobium . . . 141 General Niobium and Hydrogen Niobium and Fluorine, Chlorine, Bromine, Iodine Oxides Niobatos, Heteropoly-niobatcs, Pcrniobic Acid and the Perniobates Niobium and Sulphur Niobium and Nitrogen Niobium and Carbon.

CHAPTER VIII. Tantalum and its Alloys . . .172 Preparation of Metallic Tantalum Colloidal Tantalum Physical Properties of the Metal, Optical Properties, Arc and Spark Spectra Chemical Pro- perties Electromotive Behaviour Atomic Weight Alloys.

CHAPTER IX. Compounds of Tantalum . . . 186 General Tantalum and Hydrogen, Fluorine, Chlorine, Bromine, Iodine, Oxygen Tantalates, Hetero-tantalatcs, Pertantahc Acid and the Portantalates Tantalum and Sulphur Tantalum and Nitrogen Tantalum and Carbon, NAME INDEX ...... 207 SUBJECT INDEX ...... 214 LIST OF CHIEF ABBREVIATIONS EMPLOYED IN THE REFERENCES.

ABBREVIATED TITLE. JOURNAL.

Afhandl. Fys. Kern. . Afhandlingat i Fysik, Kemi och Mineralogi. Amer. Ghem. J. . Americaa Chemical Journal. Amer. J. Sci. American Journal of Science. Anal Us. Quim. Analea do la Sociedad Espanola Fisica y Quiniica. Analyst . The Analyst. Annalen . Justus Liebig's Annalen der Chemie. Ann. Ghim. Annales de Chimie (1719-1815, and 1914-f). Ann. Ghim. anal. Annales de Chimie analytique appliquee h> Tlndustrie, Ji F Agriculture, a la Pharmacie, et a la Biologie. Ann. Ghim. Phys. Annales de Chimie et de Physique (Paris) (1816-1913). Ann. Mines Annales des Mines. Ann. Pharm. Annalen der Pharmacie (1832-1839). Ann. Phys. Ghem. Annalen der Physik und Chemie (1819-1899). Ann. Physik Annalen der Physik (1799-1818, and 1900-h). ' Ann. Physik, Beibl . Annalen der Physik, Beiblattes. Ann. Sci. Univ. Jassy Annales scientifiques de TUniversite de Jassy. Arbeiten Kaiserl Gesundheite- amte .... Arbeiten aus dem Kaiserlichen Gesundheitsamte. Arch. exp. Pathol. PJiarmak. Archiv f ur experimented Pathologic und Pharniakologie. Arch. Pharm. Archiv der Pharmazie. Arch. Sci. phys. nat. . Archives des Sciences physique et naturelles, Geneve.

Atti Ace. Torino . Atti delia Reale Accadeniia delle Scienze di Torino.

Atti R. Accad. Lined . Atti della Reale Accademia Lincei. B.A. Reports British Association Reports. Ber. Berichte der Deutschen chemischen Gesellsehafc.

Ber. Akad. Ber. . See Sitzungsber. K. Akad. Wiss. Berlin. Ber. Deut. physikal. Ges. Berichte der Deutschen physikalischen Gesollschaft, Bull Sci. PharmacoL . Bulletin des Sciences Pharmacologiques. Bot. Zeit Botanische Zeitung. Bui Soc. Stunte Gluj. . Buletinul Societatei de Stunte din Cluj. Butt. Acad. roy. Belg. Academie royale de Belgique Bulletin de la Classj des Sciences. BuU. Acad. Sci. Gracow Bulletin international de 1' Academie des Sciences de Oacovie. Butt, de Belg. Bulletin de la Societe chimique Belgique. Ber. Deut. pharm. Ge$. Berichte der Deutschen pharmazeutischen Gesellschaft. Butt. Soc. chim. . Bulletin de la Societe chimique de France. la f de Butt. Soc. fran$. Min. . Bulletin de Societe ranQaise Mineralogie. Butt. Soc. min. de France . Bulletin de la Societe mineralogique de France. Bull U.S. Geol Survey Bulletins of the United States Geological Survey. Gentr. Min. Centralblatt fur Mineralogie. Ghem. Ind. Die Chemische Industrie. Ghem. News Chemical News. Ghem. Weekblad Ghemisch Weekblad. Chem. Zeit. Chemiker Zeitung (Cothen). Ghem. Zentr. Chemisches Zentralblatt. de 1* Academie Gompt. rend. Comptes rendus hebdomadaires des Seances des Sciences (Paris). der von CreWs Annalen . Chemische Annalen fur die Freunde Naturlelxre, L,. Crelle. JournaL Dingl poly. J. . Dingler's polytechnisches XVI VANADIUM, NIOBIUM, AND TANTALUM.

ABBREVIATED TITLE. Drude's Annalen Annalen der Physik (1900-1906).

Electroch. Met. Ind. . Electrochemical and Metallurgical Industry. Bng. and Min. J. Engineering and Mining Journal. Gazzetta .... Gazzetta chimica italiana. der Chemie. Gehlen's Allg. J. Chem. AUgemeines Journal Geol. Mag. Geological Magazine. Gilberts Annalen Annalen der Physik (1799-1824). Giom. di Scienze Naturali ed Scon Giornale di Scienae Naturali ed Economiche. Helv. Chim. Acta Helvetica Chim, Acta. Zeitschrift fur Int. Zeitsch. Metaltographie . Internationale Metallographie. der Eeichsan- Jahrb. kk. geol Reichsanst. . Jahrbuch kaiserhch-koniglichen gcologischen stalt. Jahrb. Miner. Jahrbuch fur Mineralogie. Jahresber. .... Jahresbencht uber die Fortschrittc der Chemie. Jenaische Zeitsch. Jenaische Zeitschrift fur Naturwisscnschaft. of the American Chemical J. Amer. Chem. Soc. . Journal Society. J. Chem. Soc. Journal of the Chemical Society Journal de Chimie J. Chitn. phys. . physique. J. Gasbeleuchtung Journal fur Gasbeleuchtung. J. Geology .... Journal of Geology, and J. Ind. Eng. Chem. Journal of Industrial Engineering Chemistry. of Metals. J. Inst. Metals . Journal of the Institute Journal of the J. Miner. Soc. . MIneralogical Magazine and Mineralogical Society. J. Pharm. Chim. Journal de Pharmacie et de Chimie. J. Physical Chem. Journal of Physical Chemistry. J. Physique Journal de Physique.

. Journal fur Chemie. J. prakt. Chem. praktische and Chemical of Russia J. Russ. Phys. Chem. Soc. . Journal of the Physical Society (Petrograd). J. Soc. Chem. Ind. Journal of the Society of Chemical Industry.

LanAw. Jahrb. . Landwirtschaftliche JahrbUcher. Mem. Paris Acad. Memoirs presenter par divers savants a PAcademie des Sciences de Hnstitut dc France. Mem. Coll. Sci. Kyoto. Memoirs of the College of Science, Kyoto Imperial "University. Monatsh. .... Monatshefte fur Chemie und verwandto Theile anderer Wissenschaften. Mon. scient. Moniteur scientifique. Munch. Med. Wochenschr. . Munchener Medizinische Wochenschrift.

Nature . Nature. Nuovo Cim. II nuovo Cimento.

Oesterr. Chem. Zeit. . Oesterreichische Chemiker-Zeitung.

. af Forhand' Ofvers. K. Vet.-Akad. Forh. Ofversigt Kongliga Vetenskaps-Akademiens lingar.

. Archiv fur die des Menschon und Pfluger*s Archiv gesammto Physiologic der Thiere. PJiarm. Post Pharmazeutische Post. Pharm. Zentr.-b.. Pharmazeutische Zentralhalle. and Phil. Mag Philosophical Magazine (The London, Edinburgh, Dublin). Phil. Trans. Philosophical Transactions of the E/oyal Society of London. Phys. Review Physical Review. Physikal. Zeitsch. Physikalische Zeitschrift. der und Chemie Pogg. Annalen . PoggendorfFs Annalen Physik (1824- 1877). Proc. Chem. Soc. Proceedings of the Chemical Society. Proc. K. Akad. Wetensch. Koninklijke Akademie van Wetenschappen te Amsterdam Amsterdam . Proceedings (English Version). Irish Proc. Roy. Irish Acad. . Proceedings of the Royal Academy. Proc. Roy. Phil. Soc. Glasgow Proceedings of the Royal Philosophical Society of Glasgow, the of London. Proc. Roy. Soc. . Proceedings of Royal Society of Proc. Roy. Soc. Edin. . Proceedings of the Royal Society Edinburgh. LIST OF CHIEF ABBREVIATIONS. xvii

ABBREVIATED TITLE. JOURNAL. Rec. Trav. chim. Recueil des Travaux chimiques des Pay-Bas et de la Belgique. Roy. Inst. Reports Reports of the Royal Institution. Schweigger's J. . Journal fur Chemie und Physik. Dublin Sci. Proc. Roy. Dubl. Soc. . Scientific Proceedings of the Royal Society. Sitzungsber. K. Akad. Wiss. Sitzungsberichte der Koniglich-Preussischen Akademie de Berlin. Wissenschaften zu Berlin. Sitzungsber. K. Akad. Wiss. Sitzungsberichte der Koniglich-Bayerischen Akademie Wien .... der Wissenschaften zu Wien. Techn. Jahresber. Jahresbericht uber die Leistungen der Chemischen

Trans. Amer. Electrochem. Soc. Transactions of the American Electrochemical Society. Trans. Ghem. Soc. Transactions of the Chemical Society. Trans. Inst. Min. Eng. Transactions of the Institution of Mining Engineers. Trav. et Mem. du Bureau Travaux et Memoires du Bureau International des Poids intern, des Poids et Mes. et Mesures. Naturforscher und Verh. Oes. deut. Naturforsch. Verhandlung der Gesellschaft deutscher Aerzte. Aerzte. und Chemie Wied. Annalen . Wiedemann's Annalen der Physik (1877- 1899). der Wissenschaftl. Abhandl. phys.- Wissenschaftliche Abhandlungen physikalisch-tech- tech. Reichsanst. . nischen Reichsanstalt.

Zeitsch. anal. Chem. . Zeitschrift fiir analytische Chemie. Chemie. Zeitsch. angew. Chem. . Zeitschrift fur angewandte Zeitsch. anorg. Chem. . Zeitschrift fiir anorganische Chemie. Zeitsch. Chem. . Kritische Zeitschrift fur Chemie. des Kolloide Zeitsch. Chem. 2nd. Kolloide . Zeitschrift fiir Chemie und Industrie (continued as Kolloid-Zeitschrift).

Zeitsch. Ekktrochem. . Zeitschrift fur Elektrochemie. und Zeitsch. Kryst. Min. . Zeitschrift fur Krystallographie Mineralogie. Zeitsch. Nahr. Oenuss-m. Zeitschrift fur Untersuchung der Nahrungs- und Genuss- mittel. Stochiometrie und Zeitsch. physikal. Chem. Zeitschrift fur physikalische Chemie, Verwandtschaftslehre. Chemie. Zeitsch. physiol. Chem. Hoppe-Seyler's Zeitschrift fiir physiologische Photo- Zeitsch. wiss. Photochem. Zeitschrift fur wissenschaftliche Photographic, physik, und Photochemie.

TABLE OF DATES OF ISSUE OF JOURNALS.

FOR the sake of easy reference, a list is appended of the more important journals in chronological order, giving the dates of issue of their corresponding series and volumes. In certain cases the volumes have appeared with considerable irregularity; in others it has occasionally happened that volumes begun in one calendar year have extended into the next year, even when this has not been the general habit of the series. To complicate matters still further, the title-pages in some of these latter volumes bear the later date a most illogical procedure. In such cases the volume number appears in the accom- panying columns opposite both years. In a short summary of this kind it is impossible to give full details in each case, but the foregoing remarks will serve to explain several apparent anomalies.

First series known as bulletin de PTiarmaeie. VANADIUM, NIOBIUM, AND TANTALUM. TABLE OF DATES OF ISSUE OF JOURNALS. xxi SSll 'VANADIUM, NIOBIUM, AND TANTALUM.

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VOLUME VI. PART III.

A TEXT-BOOK OF INOEGANIC CHEMISTRY

VOL. VL PART III. VANADIUM, NIOBIUM, AND TANTALUM.

CHAPTER I. GENERAL CHARACTERISTICS OF THE ELEMENTS OF GROUP V., SUBDIVISION A.

THE elements vanadium, niobium, tantalum and protoactinium constitute Subdivision A of the Fifth Group of the Periodic Classification. The general properties of Subdivision B are considered elsewhere in 1 this series. Niobium is also known as columbium. Protoactinium, also known as &#-tantalum, is a radioactive element which is also described elsewhere in this series. 2 Vanadium occurs in the first long period of the system. It is, in fact, the Eeriodicrst member of a set of nine elements (viz. vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium) which fall in the centre of the period and for which there are no analogues in the short periods. Niobium occupies an analogous position in the second long period, and tantalum in the third very long period. These three elements display the characteristic of " " properties transition elements in that they are distinctly metallic substances of high density and melting-point, associated with great hardness and other valuable mechanical properties. They display marked resemblances both in the elemental state and in their compounds to the corresponding 3 members of the A Subdivisions of Groups VI. and IV. The analogy 1 Vol. VI., Part I. (1028), p. 3. 2 Vol. VII., Part III. (1926), p. 349. 9 These subdivisions are included in the foregoing table. a 4 VANADIUM, NIOBIUM, AND TANTALUM. of the elements with the B Subdivision of Group V. is mainly restricted for instance, is on the to the pcntavalent compounds ; vanadium, to than to whole more closely parallel in properties chromium phos- case so marked phorus and arsenic. Indeed the resemblance is in this that vanadium was at one time included in the chromium family were to of elements, and analogous but erroneous formulae assigned these elements are set out its compounds. The physical properties of to the table for the in the following table. Chromium has been added 1 sake of comparison.

The metals are not readily attacked by acids, but when they pass into solution they lose their metallic character and yield derivatives Nb O and Ta . These derivatives of the acid pentoxides, V 2 5 , 2 5 2 5 include the vanadates, niobates and tantalates, which are among the commonest and most stable compounds of these metals. of the A gradual increase is observable in the electropositiveness metals in the order V Nb *Ta (increase in atomic weight), as in the case of the elements of the B Subdivision of this group, N >P > As j>Sb *Bi. It is also worthy of notice that, consistent with the usual rule, the A Subdivision on the whole is more strongly electro- these positive than the B Subdivision. The following observations show facts clearly : with increase in (a) The decrease in the acidity of the pentoxides atomic weight. Vanadium pentoxide is the most acidic of the three, in its behaviour and is somewhat comparable with chromic acid ; niobium pentoxide is much weaker, and tantalum pentoxide is still slightly weaker. These latter two acids are, in fact, extremely weak, and are comparable with titanic acid and zirconic acid (their neighbours in in Group IV.), so much so that considerable difficulty is experienced separating niobium and tantalum quantitatively from the Group IV* elements. (b) The increase in the stability of the pentahalides with increase in atomic weight. Tantalum yields a pentahalide with each of the halogens, a pentiodide of niobium is unknown, and in the case of vanadium only the pentafluoride has been prepared. The existence of

1 For a comparison of the physical properties of tantalum, tungsten, molybdenum, platinum, copper and nickel, see Balke, Ghem. Met. $ng. t 1922, 27, 1273. * This value disturbs the gradation, but it is probable that the density (12-7) used in its calculation is too high (see p. 135). GENERAL CHARACTERISTICS OF ELEMENTS. 5

tantalum pentiodide is, in fact, remarkable, because no other element in Groups IV, to VIII. displays the maximum valency of its group towards iodine. The variation in the readiness with which the penta- fluorides undergo hydrolysis is shown in the fact that when the pentoxides of vanadium, niobium, and tantalum are dissolved in hydrofluoric acid and treated with potassium fluoride under the same conditions, the

salts : following double are most readily precipitated #KF.VO 2F ;

#KF.NbOF3 ; #KF,TaF5 . With increase in the concentration of acid the double salts of hydrofluoric vanadium oxytrifluoride, VOF3 , and of niobium pentafluoride, NbF5 , can be obtained. The double fluorides and oxyfluorides of niobium and tantalum are among the commonest compounds of these two elements, and form the classical means whereby the two metals are separated from each other. Double fluorides of also analogous type are given by antimony, e.g. 2KF.SbF5 .H2O, and by

molybdenum, tungsten, etc., e.g. 2KF.MoF3 .H2 and KF.W0 2F.H 20. The double alkali fluorides formed by the metals which belong to neighbouring groups are frequently isomorphous with the corresponding double fluorides of vanadium, niobium, and tantalum, and in many instances a fluorine atom is able to undergo replacement by an oxygen atom without disturbing the crystalline form. The following are examples of pairs of isomorphous compounds : 2NH 4F.NbOF 3 and

2NH 4F.W0 2F 2 ; 3KF.NbOF 3.HF and 3KF.SnF 4.HF ; ZnF 2 .NbOF 3 . .TiF .6H the 6H 2 and ZnF 2 4 2O ; compounds 3NH 4F.NbOF 3 and 3NH 4F.TaOF 3 are isomorphous with the compounds 3NH4F.TiF 4 and and the is 3NH 4F.ZrF 4 ; compound 2KF.NbOF3.HoO isomorphous with 2KF.W0 2F 2.H 2 and with 2KF.TiF 4.H 20. As is usual with transitional elements, vanadium possesses consider-

able freedom in its valencies ; with niobium and tantalum, however, this freedom is less marked. This can be seen in the variation of the readiness displayed by the compounds of these elements to undergo

reduction : (a) Vanadates can be successively reduced in stages, and thus be made to furnish tetra-, tri- and di-valent vanadium salts by varying the reducing agent and other conditions employed. Divalent vanadium salts are obtained with nascent hydrogen ; niobates in acid solution x are reduced by nascent hydrogen approximately to the trivalent state, but no niobium salts of lower valency than five have hitherto been solutions tantalates do not isolated from the produced ; undergo reduction at all with nascent hydrogen, and, apart from the halides and the disulphide (which do not possess saline character), only penta- valeiit compounds of tantalum are known. metal the action (6) Vanadium pentoxide can be reduced to the by niobium of hydrogen at high temperatures and pressures ; pentoxide oxide whereas tantalum is more stable, and yields the trivalent Nb 2O 3 , pentoxide has not hitherto been reduced by hydrogen.

The following table summarises the more important types of compounds given by these three elements. Nitrates and carbonates of vanadium, niobium, and tantalum have not been prepared. Niobium and tantalum salts of other weak acids, are also e.g. boric acid, hydrocyanic acid, phosphoric acid, unknown, and in the case of vanadium are not well defined.

1 See p. 131. VANADIUM, NIOBIUM, AND TANTALUM.

TYPICAL COMPOUNDS OF VANADIUM, NIOBIUM, AND TANTALUM.

Pentavalent Compounds. Vanadates display isomorphism with the .12H corresponding phosphates ; e.g. Na 3P0 4 2 and Na 3V0 4 .12H aO are isomorphous. More remarkable is the isomorphism displayed by some of the double chlorovanadates with the corresponding chloro- phosphates and chloroarsenates. The isomorphism shown by vanadinite, 3Pb .PbCl 3Pb .PbCl and 3 (V0 4 ) 2 2 , mimetesite, 3 (As0 4 ) 2 2 , pyromorphite, 3Pb .PbCl led to vanadium with 3 (P0 4 ) 2 2 , being grouped phosphorus and arsenic instead of with chromium. Vanadates differ from phosphates mainly in the ease with which they undergo reduction, and in the fact that while ortf&ophosphates are more stable than pyro- or meta- phosphates, in the case of the vanadates the order of stability is reversed. Solutions of vanadates appear to contain the ions of all three types of /;/ i.e. and can to salts, [V0 4] , [V 2O 7 ]"" [V0 8 ]', and be made precipitate any one of the salts by suitable alteration of the conditions. In the case of niobates and tantalates only the m

t Also formulated 7K2 0.5Taa 6.24H2 (see p. 199). 1 See this series, Vol. VII., Part III. (1926), pp. 45, 135. GENERAL CHARACTERISTICS OF ELEMENTS. 7

best known are the 2 : 3 salts, 2R 2O5 .3V 2O 5 . Niobates and tantalates are decomposed by dilute acids, or even on being boiled in aqueous solution, with the precipitation of hydrated niobium pentoxide and tantalum pentoxide (also known as niobic acid and tantalic acid respectively), which readily form sols. Complex niobates and tantalates analogous to the polyvanadates are, however, produced by heating the pentoxides with bases in varying proportions, or by the double decomposition of the oxyfluorides of niobium and tantalum with alkalis, but the commonest salts in the cases of niobium and tantalum

are the 4 : 3 salts. It is remarkable that the alkali tantalates which con- are in this recalls tain small proportions of the base insoluble water ; the sparing solubility of potassium pyroantimonate, K 2H 2Sb 2O 7 .6H 2O. Vanadic acid differs also from niobic and tantalic acids in that it forms a number of complex acids with other acids, and a large number of heteropolyvanadates are known. Niobic acid and tantalic acid show little to oxalotanta- tendency form complex acids ; oxaloniobates and lates have, however, been prepared. The pentoxides of vanadium, niobium, and tantalum react with hydrogen peroxide to produce per-acids of the general formula HRO 4 . #H 2O. These per-acids increase in stability with increase in atomic weight. Pertantalic acid is a white solid which can be heated to 100 C. without undergoing decomposition. The oxyfluorides of these metals also take up active oxygen to yield peroxyfluorides, which are much better denned in the case of niobium and tantalum than with vanadium. Tetravalent Compounds. With reduction in valency the acid character of pentavalent vanadium gradually disappears and basic properties become manifest. The oxide VO 2 is amphoteric, and yields both tetravalent vanadium salts and vanadites. A large number of basic salts containing the vanadyl radical VO", e.g. vanadyl sulphate, are also This radical recalls the CrO MoO WO VOSO 4 , known. 2 *% 2 ", 2", " of and UO 2 radicals given by the elements the neighbouring group. and Ta0 The corresponding oxides of niobium and tantalum, Nb0 2 2 , do not give rise to salts. Trivalent Compounds. In trivalent vanadium compounds the basic character of the element is well developed, and both normal and oxy- salts of the sesquioxide V 2O 3 are well defined, e.g. vanadous sulphate, and vanadium VOC1. It has been V 2 (SO 4 ) 3 , oxymonochloride, previously mentioned that resemblances between the elements of the A and B Subdivisions of Group V. are mainly restricted to the pentavalent com- it is of interest to note that the has in pounds ; oxychloride analogues the trivalent antimony and bismuth basic chlorides, SbOCl and BiOCl. Trivalent vanadium also displays considerable analogy, however, with other trivalent transitional elements, as shown by the following : .K S0 .24H is (a) A series of vanadium alums, e.g. V 2 (S0 4 ) 3 2 4 2O, known which are isomorphous with the alums furnished by trivalent iron, chromium, cobalt, manganese and titanium. have been which (b) Double cyanides of trivalent vanadium prepared show the properties of co-ordinated compounds, e.g. K 3[V(CN)6] ; and compare with K 3[Cr(CN) 6] K 3[Fe(CN)6]. are also (c) Double thiocyanates of corresponding compositions with . known, e.g. 3KSCN.V(SCN) 3.4H 2 ; compare 3KSCN.Cr(SCN) 3 O. 4H 2 and 3KSCN.Mo(SCN) 3.4H 2 been obtained (d) Ammines of trivalent vanadium have recently 8 VANADIUM, NIOBIUM, AND TANTALUM.

3 ) 6]C1 3 ; which are closely analogous to the cobalt-ammines, e.g. [V(NH . and 3 )6]Cl 3 compare with [Cr(NH 3 )6]Cl 3 [Co(NH is from the corresponding The hydroxide V(OH) 3 distinguished in that it is wholly hydroxides of phosphorus, arsenic and antimony exist com- basic. It is insoluble in alkalis, so that there do not any to the arsemtes, pounds of vanadium which would correspond phosphites, chromites. and antimonites, or to the ferrites, aluminates, and is the In addition to the halides the only trivalent salt of niobium has S0 .Nb .6H 20. Tantalum uncertain double sulphate (NH 4 ) 2 4 2 (S04 ) 3 not given any trivalent salts. ,11-the basic Divalent Compounds. In divalent vanadium compounds these salts are character is at a maximum. As might be anticipated evolve from their extremely strong reducing agents, and hydrogen elements in the aqueous solutions. Analogy with other transitional of double divalent state is again shown : (a) In the formation cyanides, and with K 6 and K 4[Fe(CN) 6 J ; e.g. K 4[V(CN) 6 ] ; compare 4[Cr(CN) ] of some of the (6) in the isomorphism and analogous composition is with FeS0 .7H 0, and also sulphates, e.g. VS0 4.7H 2 isomorphous 4 2 .7H 0. It is of forms mixed crystals with CrS0 4.7H 2O and MgS0 4 2 of the even interest to note that the stability of the divalent salts increases members of the first long series, Ti, V, Cr, Mn, Fe, Co, Ni, titanium com- with increase in atomic weight of the metals. Divalent of pounds are also decomposed by water with evolution hydrogen, the absence of and whereas ferrous sulphate solution is stable in air, conditions of nickel sulphate solution does not oxidise under ordinary of for a exposure. Substitution of a molecule ammonium sulphate to the molecule of water in the hydrated sulphates imparts stability

. the solid double VS0 4 4 ) 2SO 4 compounds ; thus both sulphates, .(NH S0 .6H are less oxidised on 6H 2 and FeS0 4.(NH 4 ) 2 4 20, easily exposure to air than are the simple salts. Niobium and tantalum so far have not appeared to give rise to chlorides of divalent salts, but evidence for the existence of divalent 1 these elements has recently been obtained.

of the It will perhaps have been observed from the foregoing outline chemistry of vanadium, niobium, and tantalum that while these three elements form a closely related triad, vanadium undoubtedly possesses many chemical characteristics that are not displayed by either niobium or tantalum. Indeed the last two elements are so closely parallel in their reactions that considerable time elapsed before their separate identities is were definitely established, and their separation from one another still not an easy matter. The natural occurrence of the three elements under discussion is also here worthy of note, for while vanadium is almost always found in association with phosphorus and other elements of Group V., niobium and tantalum invariably occur with metals belonging to other groups, namely, iron, manganese, zirconium, titanium, and the rare earths, suggesting that the genesis of niobium and tantalum is different from that of vanadium. In this volume the occurrence, history, extraction, detection, and estimation of niobium and tantalum will be considered together in Chapter V.

1 See pp. 149 and 192. CHAPTER II. VANADIUM AND ITS ALLOYS.

Symbol, V. Atomic Weight, 50-95 (O=16).. Occurrence. Although vanadium does not occur free in nature, compounds of vanadium occur widely distributed in small quantities 1 in many rocks, and even in the ashes of plants. According to Clarke, the amount of vanadium in the earth's crust is 0-017 per cent. The corresponding figures for copper, zinc, and lead are 0-0104 per cent., 0-0039 cent., and 0-0020 cent, so that it is incorrect per per" " respectively, to refer to vanadium as a rare element, although it is true that vanadium ores from which the metal can be economically extracted occur in only a few localities. The principal industrial deposit is an impure vanadium sulphide, containing considerable quantities of free sulphur and carbonaceous matter, known as patronite, after its dis- coverer, and found in Peru. Analysis gave the following composition 2 58-79 (percent.): S, ; V, 19-53 ; SiO 2 , 6-88 ; C, 3-47 ; Fe, 2-92 ; A1 2-00 2O 3 and P 2O 5 , ; TiO 2, 1-53 ; Ni, 1-87 ; Fe 2O 3 , 0.-20 ; Mo, 0-18 ; 0-38 O, ; H 2O, 1-90. Total=99-65 per cent. Particulars of the most important ores are set out in the table on the next page. Vanadium ores are mainly of igneous origin. The vanadium sources which are of present or potential economic value can be classified 3 under several headings. I. In association with titaniferous magnetites and ilmenites. The best known deposit of this type is the Taberg iron ore in Sweden, where vanadium was first definitely discovered. II. In veins of hydrothermal origin, where the vanadium is associated with either uranium or gold. This division includes roscoelite and mottramite. III. In sulphide ores in which the mineral is associated with hydrocarbons. This class includes the patronite deposits of Peru and various vanadium-bearing asphaltites. It is probable that these asphaltites are the residuary seepage of petroleum deposits, and that they have been formed by the action of (a) hydrocarbons and (b) sulphur or hydrogen sulphide on a fairly porous rock which has been impregnated with a vanadium compound. IV. In the oxidised upper levels of certain veins of lead and copper. This class is numerous and widely distributed, and includes the various vanadates of lead, copper, zinc, etc. They may have been formed by

1 Clarke, The Data of Geochemistry, Bulletin 616, U.S. Geol Survey, Washington, 1916. 2 Hillebrand, Amer. /. Sci., 1907, [iv], 24, 141. 3 See Monograph on Vanadium Ores, Imperial Institute, London, 1924, p. 4. 10 VANADIUM, NIOBIUM, AND TANTALUM.

MINERALS CONTAINING VANADIUM. VANADIUM AND ITS ALLOYS. 11

MINERALS CONTAINING VANADIUM continued.

1 the action of percolating vanadiferous waters on compounds of lead, but their origin is doubtful. V. In sedimentary rocks. These minerals also contain oxidised vanadium, and consist of vanadates of iron, aluminium, lead, copper, etc. The carnotite deposits of Colorado are of this type. In Great Britain vanadium has been found associated with the lead ores at Wanlockhead, in the Lead Hills, Dumfriesshire, as vanadinite* at and associated with the copper deposits at Alderley Edge and Mottram St. Andrews, Cheshire, as mottramite. The latter was at one time mined and treated for its vanadium, but commercially profitable has also been supplies of this ore have now given out. Vanadium 3 in rocks at reported to occur in titaniferous iron ores at Antrim, and 4 Wicklow and Giant's Causeway. of van- The foregoing account deals with the main distribution adium the of this element in small has also ; presence very quantities been established in a variety of substances. Among these the following

1 Ditte, Compt. rend., 1904, 138, 1303. 2 Collie, J. Chem. Soc., 1889, 55, 94 3 Hodges, Chem. News, 1872, 26, 238. * Jahrb. Apjohn, ibid., 1872, 26, 183; Sonstadt, ibid., 1872, 26, 214; Thomson, Miner., 1837, 322. 12 VANADIUM, NIOBIUM, AND TANTALUM. 1 2 and may be mentioned: Clays and shales, bauxite, cryolite, rutile the and pitchblende. Vanadium is absorbed by plants from soil, 3 a from hence is found in the ash of some coals and lignites ; lignite contained San Rafael, Argentine, gave 0-63 per cent, of ash which 4 and the flue 38-22 per cent, of vanadium estimated as the pentoxide, dust from the burning of a South Yorkshire coal contained an appreci- 5 been able of vanadium. The presence of vanadium has proportion on observed in petroleum hydrocarbons, asphalt, volcanic sublimations 6 7 in technical caustic Mount Vesuvius, and meteorites ; products, e.g. 9 8 and in the blood-cells of certain ascidia, potash, sodium carbonate ; where it apparently replaces iron. of a new History. In 1801 Manuel del Rio discovered the presence a metallic substance, which was subsequently called erythronium, in lead ore found at Zimapan in Mexico. The discovery was communicated 10 to the Academic des Sciences de Paris, and the ore was examined by was Collet-Descostils, who reported, however, that the new metal impure 11 a chromium. In 1830 Sefstrom definitely established the presence of new element in a remarkably tenacious and ductile specimen of wrought iron which had been prepared from Taberg (Sm&land) ore. To this of the Sefstrom gave the name vanadium, from Vanadis, a cognomen 12 re-examined Scandinavian goddess Freia. About the same time Wohler the Zimapan ore and found del Rio's erythronium to be identical with vanadium. 13 Considerable numbers of vanadium compounds were then and examined by Berzelius, who formed the conclusion that prepared 14 vanadium belonged to the same family as chromium and molybdenum. This conclusion was subsequently shown by Roscoe to be in error, because Berzelius had been handling the oxide or nitride when he due to the thought he was dealing with the free metal. This error was to the extreme difficulty experienced in reducing vanadium compounds metal, Roscoe conducted some classical researches on vanadium from 1868 to 1870, and found that the metal forms an oxide, VO, which enters into reactions as the vanadyl radical [VO]", in an analogous also established the intimate manner to the uranyl'radical [U0 2]". He relation that exists between vanadium and the members of the nitrogen 15 family. Ditte continued the work by his extensive investigations of vanadium into the preparation and behaviour of a large number compounds.16

1 Phipson, Ghem. News, 1863, 7, 210. 2 Deville, Ann. Ohim. Phys., 1861, [iii], 61, 309, 342. a 420. Baskerville, J. Chem. Soc., 1899, 76, 666 ; Mingaye, ibid., 1904, 86, fi Kyle, Chem. News, 1892, 66, 211. Ramage, Nature, 1927, 119, 783. 6 Carobbi, Atti R. Accad. Lincei, 1926, [vi],4, 306, 382. 7 K. Vet.-Akad. 1899, Apjohn, J. Chem. Soc., 1874, 27, 104 ; Hasselberg, Ofvers. Forh., 56, 131, from /. Chem. Soc., Abs., 1901, 80, ii, 251. 8 Chem. 199 Thorpe, J. Chem. Soc., 1872, 25, 660 ; Robinson, News, 1894, 70, ; Smith, ibid., 1890, 61, 20. 9 Henze, Zeitsch. yhysiol. Chem., 1911, 72, 494. 10 Humboldt, Gilberts Anwlen, 1804, 18, 118. 11 7. Descostils, Ann. Chim. Phys., 1805, [i], 53, 260 ; Gilberts Annalen, 1822, 71, 12 SefstrSm, Pogg. Annalen, 1831, 21, 43. 13 Wohler, ibid., 1831, 21, 49. i* Berzelius, ibid., 1831, 22, 1. i* Roscoe, Phil Trans., 1868, 158, 1 ; 1869, 159, 679 ; 1870, 160, 317. is Ditte, Compt. rend., 1883, 96, 846; 1885, 101, 698, 1487; 1886, 102, 757, 918, 1019, 1105, 1310; 188G, 103, 55; 1887, 104, 902, 982, 1061, 1168, 1705, 1844; 1887, 105, 813, 1067; 1888,io6,270. VANADIUM AND ITS ALLOYS. 13

Roscoe obtained his vanadium from the residual lime precipitate which was thrown down during the extraction of cobalt from the copper deposits at Alderley Edge and Mottram St. Andrews in Cheshire, England. Although the total amount of vanadium in the ore was only small (the lime precipitate contained about 2 per cent, of vanadium), these deposits for a long time formed an important source of supply of vanadium compounds. Until the year 1900 the only industrial applications of vanadium compounds lay in their employment as catalysts in the manufacture of aniline-black, and in their use as mordants in dyeing and calico-printing. The Cheshire source for these purposes was supplemented and later displaced by supplies obtained from the Spanish lead mines and from the basic steel slags produced at Le Creusot Steel Works in France. It was found that these slags 1 contained over 1 per cent, of vanadium. The industrial application of vanadium received its main impetus, however, when the metal entered the domain of metallurgy. In 1893 Moissan applied his electric furnace to the making of alloys of vanadium, and produced ferrovanadium in large quantity. The mechanical 2 properties of vanadium steels were noted by Helouis in 1896, but were first thoroughly investigated at Sheffield, England, by Professor Arnold in 1900 (see p. 26), whose work was followed by that of Sankey and 3 Smith in 1904. The discovery of the vast Peruvian deposits in 1905 was followed by the successful preparation from them of a ferrovanadium alloy which could readily be employed in the manufacture of vanadium steels. This process now absorbs nearly all the world's production of vanadium. Commercial Sources of Vanadium. The world's most important vanadium supply comes from the deposits of patronite in Peru. The ore occurs in the coal deposits at Minas Raga, and is essentially a sulphide of vanadium containing 10 per cent, or more of vanadium 4 and 30 per cent, or more of free sulphur. The substance is hard, and has the appearance of a black, slaty coal. The surrounding earth is impregnated with vanadates or other vanadium compounds, and contains numerous deposits of asphaltites, the ash from which yields from 20 to 40 per cent, of vanadium pentoxide. The patronite deposits are supposed to have been formed by the upward movement of asphaltic petroleum and its subsequent evaporation, the vanadium being derived from the vegetable matter which gave rise to the petroleum. The mines at Minas Raga are the highest in the world, and are about 16,000 feet above sea-level. The ore is here submitted to a preliminary roast- ing, whereby the vanadium content is increased to about 20 per cent, with almost total elimination of sulphur. The mines are connected by rail to Callao, whence shipment of the material takes place to the United States for further treatment. A secondary source of supply lies in the carnotite deposits of Colorado and Utah. The vanadium content in these is very low, being only about 1 per cent, or even less, and the ore is really worked for its radium and uranium content, the vanadium forming a by-product. The deposits of carnotite are considerable.

1 Witz and Osmond, Chem. News, 1882, 46, 47. 2 Helouis, J. Soc. Chem. Ind., Abs. 9 1896, 15, 657. 8 Sankey and Smith, J. Soc. Chem. 2nd., 1905, 24, 444. * Hillebrand, Amer. J. Sci., 1907, [iv], 24, 141. 14 VANADIUM, NIOBIUM, AND TANTALUM. Commercial sources of vanadium which are now disused arc the near Santa Marta roscoelite deposits in Colorado, the vanadinite deposits Cheshire and 1 in Spain, and the mottramite deposits in Shrewsbury. It is of some interest to note that efforts are being made to extract the vanadium from the Taberg iron ores or slags, in which vanadium was 2 3 originally discovered, and from the Rhodesian deposits.

Extraction of Vanadium.

For industrial purposes vanadium is not required in the elemental state. More than 90 per cent, of the world's production of vanadium is used in the manufacture of special steels, for which purpose an iron- from 30 to 40 vanadium alloy, known as ferrovanadium, containing of manufacture of per cent, of vanadium, is marketed. The method with the this alloy from vanadium-bearing ores varies considerably The composition of the ore and the value of the by-products. process is conveniently divided into two stages : of crude I. The preparation of a complex mixture of vanadates or vanadium pentoxide or of crude iron vanadate. II. The conversion of these products into the iron-vanadium alloy.

I. The Preparation of a Complex Mixture of Vanadates or of Crude Vanadium Pentoxide or of Crude Iron Vanadate. A. Dry Process. The ore is roasted in a reverberatory furnace of about eighty tons capacity. The time occupied in passing the ore through the furnace is about two days, this time being necessary in order to burn off the asphaltic material which, together with any free sulphur also present, renders the addition of fuel unnecessary except towards the end of the heating. The roasted product contains from 40 to of 50 per cent, of vanadium pentoxide and not more than 0'5 per cent, iron sulphur, the rest being made up of silica, alumina, lime, magnesia, and nickel in varying proportions. This material is mixed with suitable fluxes and subjected to a matte smelting in a second reverberatory furnace. A matte is formed of all the foreign metals present in the ore, and a supernatant slag is produced, which contains all the vanadium combined with the gangue material (silica, lime, alumina, magnesia) as mixed vanadates.4 B. Wet Processes. These vary considerably in detail according to the nature and amount of constituents other than vanadium in the ore. An outline of the operations involved in the case of patronite is as follows : The ore is roasted with common salt or sodium carbonate and then extracted either (a) with water to give an alkaline solution of sodium vanadate and soluble vanadates of other metals, any lead, zinc, copper, etc., being left in the residue; or (b) with sulphuric acid to produce a solution of vanadyl sulphate. Acid extraction is usually employed when the vanadium content of the material is low. The alkaline extract from (a) is treated with excess of sodium carbonate in order to precipitate calcium and aluminium, after removal of which, 1 Smith, /. SOG. Chem. 2nd., 1901, 20, 1183; Gerland, ibid., 1901, 20, 1188; Gin, Trans. Amer. Electrochem. Soc., 1909, 16, 409. 2 See Chem. Met. Mng., 1922, 27, 321; Amer. Chem., Abs. 9 1925, 19, 3234; 1927, 21, 2638 ; Kjellberg, Eng. and Min. J., 1927, 123, 521. 3 Walker, ibid., 1928, 125, 733. 4 Saklatwalla, Trans. Amer. Mectrochem. Soc. 9 1920, 37, 341. VANADIUM AND ITS ALLOYS. 15

addition of ferrous sulphate throws down a precipitate of iron vanadate 1 of uncertain composition. The acid solution of vanadyl sulphate from (b) is either evaporated to a cake and the residue calcined to vanadium pentoxide, or the solution is treated directly with oxidising agents, e.g. hypochlorites, whereupon precipitation of vanadium pentoxide takes place :

2VOS0 4 +C1 2 +3H 2O=:V 2O 5 +2H 2SO 4 +2HCL For the treatment of carnotite several methods are available. The method recommended by the United States Bureau of Mines 2 is as follows : The ore is leached with concentrated nitric acid at 100 C., neutralised with caustic soda, and barium chloride and sulphuric acid added to the solution to precipitate the radium as barium-radium sulphate. The precipitate settles in three or four days, after which time the clear liquid is decanted into tanks and is treated with excess of boiling sodium carbonate solution in order to precipitate any iron, aluminium and chromium present. The solution now contains sodium uranyl carbonate and sodium vanadate. It is nearly neutralised with nitric acid, and caustic soda is added in sufficient quantity to precipitate the uranium as sodium uranate. After filtering, the remaining solution is neutralised with nitric acid and ferrous sulphate added, whereupon iron vanadate is thrown down. By this method it is claimed that 90 per cent, of the radium, all the uranium, and 50 per cent, of the vanadium in the carnotite are recovered. Electrolytic methods for the separation of vanadates of the metals have also been suggested, but do not appear to have come into general use.3

II. The Conversion of the Products of the Previous Stage into Iron-Vanadium Alloys. The preparation of the iron-vanadium alloy from the crude vanadates obtained in any of the foregoing processes is carried out almost entirely at Bridgville, Pennsylvania. The process consists in reduction of the material with carbon in the electric furnace. Three graphite rods of 12-inch diameter are suspended in an intimate mixture of vanadium compound, iron ore or scale, fluxing agent (lime or fluorspar), and coke, contained in a cast-iron furnace lined successively with bricks and carbon blocks. The material is moved by a worm-conveyor into the high-temperature zone, and thence is immediately removed in order to prevent reoxidation of the vanadium. Two tap-holes are provided, 4 one for alloy and one for slag, and continuous feed is employed. A good sample of the alloy produced in this manner gave the following analysis :

1 499. Saklatwalla, loc. cit. ; Bleecker, Met. Chem. Eng., 1911, 9, 2 also Met. See U.S. Bureau of Mines, Bulletin 104, 1915, p. 30 ; Conley, Chem. Eng., 429 Thews and 1919, 20, 514 ; d'Aguiar, ibid., 1921, 25, 825 ; Doerner, ibid., 1924, 31, ; Heinle, J. Ind. Eng. Chem., 1923, 15, 1159. 3 Bleecker, loc. cit. 4 J. Ind. 968 Scott, The Engineer, 1923, 136, 636 ; Saklatwalla, Eng. Chem., 1922, 14, ; Electrical World, 1923, 81, 452, 16 VANADIUM, NIOBIUM, AND TANTALUM.

The usual vanadium content of commercial ferrovanadium is, however, 1 between 30 and 40 per cent. Carbon has a great tendency to combine with vanadium to form carbides, the presence of which in the alloy renders it unsuitable for use in steel manufacture. The successful employment of carbon as the reducing agent is in fact quite recent. Formerly silicon, an iron- it was silicon alloy, or aluminium was used in place of carbon, but difficult to obtain a product which was free from silicon or aluminium, 2 and considerable loss of vanadium took place in the slags. Modifications of the Goldschmidt thermite process may also be employed for the preparation of the iron-vanadium alloy. The crushed vanadates or vanadium pentoxide are mixed with the necessary amount a of iron scalings or turnings and fluxes, and introduced into gas-fired a open-hearth furnace or into an iron crucible provided with refractory reactions lining and previously heated to redness. The taking place

are :

(i) 3V 2 5 +K>A1=6V+5A1 2O 3 .

(ii) Fe 2 3 +2Al =2Fe+Al 2 3 .

With a vertical-shaft furnace a much higher temperature, 2500 to crucible. 2800 C., and a much larger output can be obtained than with a A furnace 9 feet 3 inches high and 4 feet 6 inches wide will produce " 3 125,000 Ib. of alloy in one run." Preparation of Vanadium. There is no demand for pure vanadium, and the isolation of the metal is therefore not an industrial is attended with con- process. Even on the small scale the operation siderable difficulty, owing to the very high temperature necessary for the reduction of vanadium compounds and the tendency for reoxidation to take place. The following methods have given products of variable purity : the Process. Vanadium (i) Modifications of Goldschmidt pentoxide, with twice its of an of the rare earths V 2 5 , is mixed weight alloy obtained in the manufacture of thorium nitrate, and consisting" roughly of 45 per cent, cerium, 20 per cent, lanthanum, 15 per cent. didymium," and about 20 per cent, of other rare metals. The reaction is carried out in a magnesia-lined crucible and is started with a firing mixture of barium peroxide, potassium chlorate, and aluminium powder. Con- 4 siderable evolution of heat takes place. It is claimed that vanadium 5 of 99-7 per cent, purity can be obtained by this method. Samples of vanadium, which in some cases were 100 per cent, pure, have recently been obtained by reducing the pentoxide with a mixture of finely milled calcium and calcium chloride in a bomb heated electrically for an hour at 900 to 950 C. The presence of hydrogen or carbon should be avoided, and the operation is best conducted in vacuo.* 7 Vanadium pentoxide is not easily reduced by means of aluminium,

1 For analyses of other samples see Monograph on Vanadium Ores, Imperial Institute, Revue de 135. London, 1924, p. 13 ; Pourment, Metallurgie, 1926, 23, 2 J. Soc. Chem. 367. Saklatwalla, loc. ciL ; Vogel, Ind., 1924, 43, 3 542 Trans. Amer. Matignon and Monnet, Compt. rend., 1902, 134, ; Saklatwalla, Met. 1141. fflectrochem. Soc., 1920, 37, 341 ; Ohem. and Eng. 9 1922, 26, * 380. Muthmann, Chem. Zeit., 1904, 28, 506 ; Weiss and Aiohel, Annalen, 1904, 337, 5 Muthmann, Weiss, and Riedelbaueh, Annalen, 1907, 355, 59. Marden and Rich, J. Ind. Eng. Chem., 1927, 19, 786. 7 Goldschmidt, Zeitsch. EkUmkm*, 1898, 4, 494; Hittorf, Physical Zeitsch.> 1903, 4, 196. VANADIUM AND ITS ALLOYS. 17

which also tends to alloy with the product. 1 Even with the addition of carbon, calcium fluoride, or calcium carbide to the reaction mixture, complete reduction does not ensue. 2 Meyer and Backa obtained vanadium of only 93-5 per cent, purity using vanadium pentoxide 3 and aluminium as in the Goldschmidt process. Vogel and Tammann claim to have prepared vanadium of more than 99-07 per cent, purity the by same method, but did not ascertain the conditions necessary 4 for success, Ruff and Martin prepared 95 per cent, pure vanadium vanadium 5 by using trioxide, V 2O 3 , and aluminium. of The use calcium in place of the rare-earth alloy as the reducing agent gives a product containing from 91 to 93 per cent, of vanadium, while a mixture of calcium and aluminium produces 94-5 per cent, pure 6 vanadium. Lithium has been used as reducing agent, vanadium of 99 per cent, purity being claimed. 7 (ii) Reduction of Chlorides. Roscoe reduced vanadium dichloride, VC1 at a red heat with 2 , bright hydrogen, every precaution being taken to prevent the entry of moisture and oxygen into the apparatus. The product was 95-8 per cent, pure metal, the impurity being mainly hydrogen. This method is of interest in that by its means metallic 8 vanadium was first obtained; the process is, however, very slow. Reduction of the chlorides of vanadium by means of sodium gives a 9 product of doubtful purity. Billy claims to have prepared pure vanadium the of by passing vapour vanadium tetrachloride, VC1 4, over 10 sodium hydride, prepared in situ, at 400 C. (iii) Electrolytic Reduction at High Temperatures. The deposition of metallic vanadium by electrolysis of a solution of a vanadium salt at 11 ordinary pressures has not hitherto proved successful. The reason is that vanadium compounds of low valency frequently decompose water with evolution of hydrogen and undergo oxidation with increase of valency, so that the formation of the free metal does not ensue. The electrolytic isolation of other strongly electropositive metals is attended with the same difficulty. Electrolysis of anhydrous fused vanadium salts or reduction of vanadium oxides in the electric furnace can, however, be 12 successfully employed. Thus, the metal has been obtained by electrolysing vanadium trioxide or pentoxide dissolved in a bath of molten vanadium tetrafluoride and calcium fluoride. The anode is made of carbon and the cathode of lead. A lead-vanadium alloy is obtained from which the lead is subsequently volatilised. This process is similar

1 Moissan, Compt. rend., 1896, 122, 1297. 2 Prandtl and Koppel and Kaufmann, Zeilsch. anorg. Chem., 1905, 45, 352 ; Bleyer, Ber., 1910, 43, 2602. 3 Meyer and Backa, Zeitsch. anorg. Chem., 1924, 135, 177. 4 Vogel and Tammann, ibid., 1909, 64, 225. 5 Prandtl and Ruff and Martin, Zeitech. angew. Chem., 1912, 25, 49 ; but compare Manz, Zeitsch. anorg. Chem., 1912, 79, 209. 6 Prandtl and Bleyer, ibid., 1909, 64, 217. 7 - Baughman, Trans. Amer. Electrochem. Soc., 1923, 43, 305, but details of his method are not given. 8 Roscoe, PHI. Trans., 1869, 159, 679. 9 cit. No. 10, 13 ; Roscoe, loc. ; Setterberg, Ofvers. K. Vet.-Akad. Idrh., 1882, 39, and Trans. Amer. Prandtl and Manz, Zeitsch. anorg. Chem., 1912, 79, 209 , Hunter Jones, Mectrochem. Soc., 1923, 44, 28. 10 Billy, Compt. rend., 1914, 158, 578. 11 Fischer, Trans. Amer. Mectrochem. 8oc., 1916, 30, 175. 12 Compare Marden and Rich, J. Ind, JEng. Chem., 1927, 19, 786. - VOL. vi. : in. 2 18 VANADIUM, NIOBIUM, AND TANTALUM. to the Heroult method for the extraction of aluminium. 1 Gin electrolysed molten calcium fluoride using a steel cathode and an anode composed of a mixture of carbon and vanadium trioxide, V 2 3 . Vanadium trifluoride is formed on the anode, passes into the molten electrolyte 2 and is then decomposed, the vanadium being deposited on the cathode. 3 Beckman electrolysed a vanadium oxide in a bath of molten lime. Electric (iv) Reduction of Oxides of Vanadium in the Furnace. The reduction of vanadium pentoxide or trioxide by means of carbon 4 yields a product which contains some of the carbon as carbide. Friederich and Sittig were unable to obtain a sample containing more than 82 per cent, of vanadium when they reduced a mixture of vanadous in an of 5 oxide, V 2O 3 , and carbon atmosphere hydrogen. Using sugar- charcoal and carrying out the reduction in an atmosphere of hydrogen in an electric furnace, Moissan was able to reduce the carbon content 6 to 4*4 per cent. Ruff and Martin obtained 98-11 per cent, vanadium by heating to 1950 C. a mixture of vanadium carbide and vanadium 7 trioxide pressed into a rod in a zirconia crucible. The reduction of vanadium trioxide has also been effected by passing an electric 8 current through rods of the material in a good vacuum, and by the action of hydrogen at a pressure of 5 atmospheres and a temperature of 2500 C, 9 The isolation of vanadium can be effected on a very small scale, suitable as a lecture experiment, by passing an electric current through a wire filament in platinum immersed vanadium oxytrichloride, VOC1 3 , 10 either in vacuo or in an atmosphere of hydrogen. The metal is obtained as a smooth, silver-grey deposit.

Physical Properties of Vanadium.

The reported data are not always in good agreement owing to the fact that vanadium in varying conditions of purity has been used for the determination of constants by the various investigators. Vanadium has been variously described as : a bright grey metal which appears as silver-white under the u a lustrous, crystals microscope ; grey powder in which glistening, needle-like crystals can be seen with 12 the naked eye ; small crystals, of differing crystalline form, which appear blue or olive-green on the surface, and which possess flat, 13 glistening faces ; silvery, well-formed crystals of twin rhomboids, to the u similar belonging hexagonal system ; in appearance to cast

1 Battghman, Trans. Amer. Electrochem. Soc., 1909, 16, 439. 8 Zeitsch. Trans. Gin, EleJctrochem., 1903, 9, 831 ; Baughman, Amer. Mectrochem. Soc., 1909, 16, 442. 3 Beckman, ibid., 1911, 19, 171. 4 Moissan, Compt. rend., 1893, 116, 1225. 6 Friedericli and Sittig, Zeitsch. anorg. Cliem., 1925, 143, 303. 6 Moissan, Compt. rend., 1896, 122, 1297. 7 Ruff and Martin, Zeitsch. angew. Ghem., 1912, 25, 49. 8 von Bolton, Zeitsch. Elektrochern., 1905, u, 45; compare Baughman, Trans. Amer.

Electrochem. Soc. } 1923, 43, 310. Wartenberg, Broy, and Reinieke, Zeitsch. Elektrochem., 1923, 29, 214. 10 Edson and Mclntosh, Trans. Roy. Soc. Canada, 1915, [iii], 9, 81. 11 Roscoe, Phil. Trans., 1869, 159, 679. 12 Setterberg, Ofvers. K. Vet.-Akad. Forh., 1882, 39, No. 10, 13. 13 Brogger and Plink, Zeitsch. Kryst. Min., 1884, 9, 232. 14 Weiss and Aichel, Annalen, 1904, 337, 380, VANADIUM AND ITS ALLOYS. 19

1 iron which is rich in carbon, and sometimes forms long prisms. X-ray analysis shows, however, that vanadium possesses a body-centred cubic lattice of side to 3-04 crystal equal A ; the distance between the nearest 2 atoms is 2-63 A. The metal is rather brittle but its extremely hard ; hardness on Mohr's scale is 7-5, so that it cannot be scratched even by quartz 3 or steel. The density of pure vanadium is 6-0 at 22 C., 6-025 at 15 C. 4 Other reported figures vary with the purity of the sample : C. for 5 6 7 5-688 at 18*7 a specimen 98 per cent, pure; 5-53, 5-5 at 15 C., 8 9 10 5-8 at 20 C., 5-97 and 5-987 at 20 C. for samples about 95 per cent, pure. The melting-point of the purest vanadium obtainable is given 11 12 13 as 1720 :20 C. Other reported figures are 1650 C., 1680 C., 1700 14 1715 15 16 17 about C., C., 175030 C., below 1760 C. The specific 18 heat of vanadium is 0-120 between 20 and 100 C., 0-1153 between 19 20 21 and 100 C., 0-1240, 0-1259, 0-1235 to 0-1258. 22 The metal is even non-magnetic ; when subjected to a temperature of 259 C. it feeble indication 23 gives only very of ferromagnetism. The specific electrical resistance of cold-worked vanadium metal is 26 x 10~ 6 ohms cc. at 20 C. the per ; temperature coefficient of resistance between 20 and 150 C, is 0-0028. Vanadium can be cold-rolled into wire ; it has a tendency to become harder when so treated and annealing is beneficial. Photomicrographs of vanadium metal are given in the reference cited. 24 The refractive index of vanadium is 3*03, the coefficient of absorption 3-51, and the reflexion capacity 57-5 per cent, for yellow light 25 of wave-length A=5790. Vanadium compounds do not impart any coloration to the ordinary Bunsen flame, and do not furnish any characteristic line spectra even in the oxyacetylene flame. The flame produced between carbon electrodes consists of a reddish-purple core 26 with a yellowish-green shell and a red edge.

1 Muthmann, Weiss, and Riedelbaueh, Annalen, 1907, 355, 58. 2 Hull, Phys. Review, 1922, 20, 113. 3 Harden and Rich, 7. 2nd. Eng. Chem., 1927, 19, 788. This figure was given by tho cold- worked metal. 4 Muthmann, Weiss, and Riedelbauch, loc. cit. 5 Ruff and Martin, Zeitsch. angew. Chem., 1912, 25, 49. 6 Hunter and Jones, Trans. Amer. Electrochem. Soc., 1923, 44, 28. 7 Roscoe, J. Chem. Soc., 1870, 23, 357. 8 9 Moissan, Oompt. rend., 1896, 122, 1297. Hunter and Jones, loc. cit. 10 Prandtl and Manz, Zeitsch. anorg. Chew,., 1912, 79, 209. 11 7. Burgess and Waltenburg, Washington Acad. Sci., 1913, 3, 371 ; Baughman, Trans. Amer. Electrochem. 8oc., 1923, 43, 286. 12 Arsem, ibid., 1923, 43, 313. 18 von Bolton, Zeitsch. Ekktrochem., 1905, n, 45 (determined by the Lummer photo- metric method). 14 Harden and Rich, loc. cit. 16 Ruff and Martin, loc. cit. (determined by extrapolation). 16 Vogel and Tammann, Zeitsch. anorg. Ghern., 1908, 58, 73. 17 Slade and Higson, J. Chem. Soc., 1919, 115, 209. 18 Harden and Rich, loc. cit. is Hache, Sitzungsber. K. AJcad. Wiss. Wien, 1897, 106, 590. 20 Huthmann, Weiss, and Riedelbauch, loc. cit 21 Setterberg, Ofvers. K. Vet.-Akad. Idrh., 1882, 39, No. 10, 21. 22 Matignon and Monnet, Gompt. rend., 1902, 134, 542. 23 122 Weiss Honda, Ann. Physik, 1910, 32, 1027 ; Loring, Chem. News, 1914, 109, ; 2146. and Onnes, Compt. rend., 1910, 150, 687 ; Weiss and Collet, ibid., 1924, 178, 24 Harden and Rich, J. 2nd. Eng. Chem., 1927, 19, 787. 25 Wartenberg, Verh. Deut. physical Qes., 1910, 12, 105. 26 Hott, Trans. Amer. Electrochem. Soc.> 1917, 31, 272. 20 VANADIUM, NIOBIUM, AND TANTALUM.

x Lockyer and Baxendall investigated the arc spectrum of vanadium by volatilising vanadium chloride and vanadium oxide between poles of pure silver. Using a Rowland grating, over 650 lines were obtained in the region between A 3887 and A 4932, the more intense of which are indicated in the following table :

ARC SPECTRUM OF VANADIUM.2

The most intense lines produced by vanadium in the spark spectrum 3 are set out in the table on the opposite page. The most intense lines in the spark spectrum of a metal are not necessarily the most persistent when solutions containing the metal 4 in gradually increasing dilution are sparked. By photographing the spectra given by solutions containing one gram of the metal, usually in the form of its chloride, in every 100, 1000, 10,000, and 100,000 parts of solution, it has been possible to draw up a table (p. 22) showing the

1 Lockyer and Baxendall, Proc. Eoy. 8oc. t 1901, 68, 189. 2 Earlier determinations than the above are those of Thalen (Nova acta regies societatis scientiaruin, Upsala, 1868, [iii], 6, 9, 36), Hasselberg (Konglinga Svenska vetenkaps J. Academien Handlingar, 1899, 32, 2), and Rowland and Harrison (Astrophys. 9 1900, 7, 273, 373). More recently a fresh determination in the region I 2207 to A 4606 has been carried out by Ludwig (Zeitsch. wiss. Photochem., 1917, 16, 157). Additional lines in the arc to U.S. Bureau spectrum from A 4500 A 9522 have been tabulated by Kiess and Meggers ( of Standards, Bulletin, 1920, 16, 51), and the spectra produced by volatilisation of the metal in the eloctric 86 furnace have been investigated by King (Astrophys. /., 1915, 41, ; 1924, 60, 282), Hemsalech (Phil. Mag., 1920, [vi], 39, 241), and by Gieseler and Grotrian (Zeitsch. fur pliysik., 1924, 25, 342). Regularities in the structure of the vanadium arc spectrum have also recently been studied by Kiess and Meggers (J. Washington Acad.

Science, 1923, 13, 317 ; 1924, 14, 151), Beohert and Sommer (Zeitsch. fur physik., 1925, 31, 1), and Meggers (ibid., 1925, 33, 509). 3 Exner and Haschek, Sitzungsber. K. Akad. Wiss. Wien, 1898, 107, [2]; see also Eppley, J. Franklin Inst., 1926, 201, 333, and Russell, Astrophys. J., 1927, 66, 184, 233. * Hartley, Proc. Roy. Soc., A. 1902, 69, 283. VANADIUM AND ITS ALLOYS. 21

SPARK SPECTRUM OF VANADIUM.

most persistent lines under these conditions. The application of the knowledge of the residuary lines given by a metal renders possible its estimation in a solution of unknown strength. The quantitative 2 spectrum of vanadium is as shown in the table on the next page. The following marks of identification are employed in the table : <=seen with 1-0 per cent, solutions but not with 0*1 per cent, solutions. 0*1 O'Ol 0-01 0-001

Measurements of wave-lengths in X-ray high frequency spectra are 3 given in the references cited. The electron configuration of vanadium 4 atoms has been investigated. Attempts have been made to bombard the vanadium atom with the view to obtaining hydrogen nuclei, but 6 without result.5 Vanadium is not radioactive.

1 There is a general want of uniformity in the standards that have been adopted by different observers for measuring the relative intensities of spectral lines. The intensity refer to a common standard figures given in this table and in the preceding table do not of measurement. 2 Pollok, Proc. Roy. Dublin JSoc., 1909, n, 331; Baly," Spectroscopy (Longmans, London), 1927, 2, 144; see also de Gramont, Compt. rend., 1920, 171, 1106. 3 K series : Hjalmar, Phil Mag., 1921, 41, 675 ; Dolejsek, Compt. rend., 1922, 174, Zeitsch. 159 ; Coster, ibid., 1924, 25, 441 ; Siegbahn and Dolejsek, PhysiJc, 1922, 10, 210 83 ; Walter, ibid., 1924, 30, 357 ; Lindh, ibid., 1925, 31, ; Larsson, ibid.,lQ26, 35 Soc. 159 Hendricks and <7. 401 ; Levi, Trans. Hoy. Canada, 1924, [iii], 18, ; Wyckoff, Phys. 1007. Chem., 1927, 31, 703. L series : Thorseus, Phil. Mag., 1926, [vii], 2, 4 and Samuel and Markowitz, Zeitsch. Physilc, 1926, 38, 22 ; Lessheim Samuel, 606, 655. ibid,, 1926, 40, 220 ; Gibbs and White, Phys. Eeview, 1927, 29, 359, 426, 5 Kirsch and Pettersen, Phil Mag., 1924, [vi], 47, 500. 6 Strong, Amer. Chem. J., 1909, 42, 147. 22 VANADIUM, NIOBIUM, AND TANTALUM.

QUANTITATIVE SPECTRUM OF VANADIUM.

Physiological Action. Vanadium compounds are poisonous when 1 taken internally. The usual symptoms are paralysis, convulsions, lowering of the body temperature, and feeble pulse. The fatal dose in the case of a rabbit is between 0*00918 and 0*01466 gram. Workmen exposed to fumes of vanadium compounds, especially those engaged on ore-reduction plants, are said to be susceptible to vanadium poisoning, but this has been denied. 2 Vanadium compounds have been shown

1 Proc. 40 Proc. Priestley, Roy. Soc., 1875-76, 24, ,- Jackson, Amer. Physiol. Soc. t 1911, 23-24. 2 Lees, Eng. and Min. J., 1911, 92, 99. VANADIUM AND ITS ALLOYS. 23

1 to possess antiseptic action, and in suitable form to be useful in 2 are at used in 3 medicine ; they not, however, present medicine. Chemical Properties of Vanadium. Pure vanadium is stable and retains its lustre in damp air. On being rapidly heated in a stream of oxygen the powdered metal burns, forming vanadium pent- the reaction 4 At a oxide, V 2 5 ; is, however, incomplete. bright red heat the metal combines with nitrogen to form a nitride. In excess of to a is chlorine, vanadium burns form tetrachloride, VC1 4 , which also produced by the action of carbonyl, sulphuryl, thionyl, and sulphur chlorides at 600 C.5 When heated in hydrogen the gas is absorbed. 6 Vanadium is not attacked by solutions of alkali chlorides, bromine water, or cold hydrochloric acid, whether dilute or concentrated, Hydrochloric acid gas, however, at 300 to 400 C. gives rise to the 7 . is attacked trichloride, VC1 3 Vanadium slowly by hydrofluoric acid and by hot, concentrated sulphuric acid. A specimen of vanadium which contained 8'66 per cent, of carbon and 1-6 per cent, of other impurities, when treated with concentrated sulphuric acid at 330 C., with evolution of dioxide. gave vanadium pentoxide, V 2O5 , sulphur the was but this At lower temperatures dioxide, V0 2, formed, was 8 converted into the pentoxide when the temperature was raised, thus : = (i) 2V+4H 2S0 4 2V0 2 +4S0 2 +4H 20.

(ii)

Vanadium is readily attacked in the cold by dilute nitric acid and by concentrated nitric acid or aqua-regia, giving vanadic acid. This latter acid is also formed by the action of other oxidising agents on vanadium, as, for example, chloric acid, perchloric acid, bromic acid, potassium iodate. On being fused with sodium carbonate, caustic potash, or potassium nitrate, vanadates of sodium or potassium are produced. Vanadium reduces solutions of mercuric chloride, cupric chloride, and ferric chloride to mercurous chloride, cuprous chloride, and ferrous the metal from solutions chloride respectively, and precipitates of gold chloride, silver nitrate, platinum chloride, iridium chloride. Carbon monoxide attacks vanadium between 500 and 800 C. with the forma- of a carbide. Glass and vessels absorb vanadium at tion porcelain " 9 be rendered as in the high temperatures. The metal can passive," chromic nitric case of iron, by immersion in oxidising agents, e.g. acid, the metal the anode in an bath of various acid, or by making electrolytic " " salts.10 Cathodic treatment reconverts the vanadium to the active

1 309 Pharmaceutische Witz and Osmond, Bull Soc. Mm., 1886, [ii], 45, j Weber, Zeitung, 1898, 43, 667. ,.,., 2 251 and Larmuth, J. Anat. Physiol, 1877, n, ; Gamgee Laarnmth, ibid., 1877, see mer. Soc. Chem. 1373 R ; Fournier, A Chem. Abs., II, 235 ; Anon., J. 2nd., 1922, 41, 1601 ; Ergebnisse Physiol, 1925, 24, 474 ; 1922, 16, 4283 ; Winkler, ibid., 1925, 19, Spiro, Levaditi and others, rend., 1928, 187, 434. U.S. Pat. 1607196 (1927) ; Compt. 3 40. Compare Partridge, /. Ind. Eng. Chem., 1929, 21, 4 Muthmann, Weiss, and Riedelbauch., Annalen, 1907, 355, 58. s Meyer and Backa, Zeitsch. anorg. Chem., 1924, 135* 177. 58 Prandtl and Muthmann, Weiss, and Riedelbauch, Annalen, 1907, 355, ; Manz, and Sieverts, Ber., 1926, 59, 289. Zeitsch. anorg. Chem., 1912, 79, 209 ; Huber, Kirschfeld, 7 Meyer and Backa, loo. cit. * 99 Marino, Zeitsch. anorg. Chem., 1904, 39, 152 ; compare p. etseq. 9 Roscoe, /. Chem. Soc., 1870, 23, 358. 929 Thiel and 10 Muthmann and IHuenberger, Zeitsch. EkMrochem., 1904, 10, ; 15. Hammerschmidt, Zeitsch. anorg. Chem. 9 1924, 132, M VAHADIUM, OTOBltJM, AND TANTALUM.

1 state. Marino, however, could not effect this change, but this may have been due to differences in the purity of the vanadium specimens used. the Atomic Weight of Vanadium. The early investigations into of the atomic weight of vanadium provide a very interesting example Berzelius in 1831 2 application of Mitscherlich's Law of Isomorphism. in at a obtained the value 68*5 by (a) reducing vanadic acid hydrogen to vanadic acid. Berzelius red heat, (b) reconverting the reduced oxide assumed in his calculations that the formula for vanadic acid was VO 3 . 3 had been In 1868 Roscoe pointed out that the following minerals 4 time shown by Rammelsberg to be isomorphous, but were not at that represented by analogous formulae :

.PbCl . (i) Vanadinite, 3Pb 3 (V0 3 ) 2 2

.PbCl . (ii) Mimetesite, 3Pb 3 (As0 4 ) 2 2

.PbCl . (iii) Pyromorphite, 3Pb 3 (P0 4 ) 2 2 If the Law of Isomorphism held good in this instance, the formula for vanadinite should be similar to those written down for the other thus be O minerals. The formula for vanadic anhydride should V 3 5 , for and arsenic corresponding to P 2 5 and As 2O6 phosphoric anhydride Berzelius. Roscoe was respectively, and not V0 3 as was supposed by as able to show that the anhydride was correctly represented V 2 5 , and that the substance regarded by Berzelius as metallic vanadium was in fact an oxide, VO. The formula for vanadinite thus becomes ,PbCl and the atomic of vanadium as 3Pb 3 (V0 4 ) 2 2 , weight originally determined becomes 68-5 160 or 52-5. Even this figure is incorrect, 5 because the materials employed were not pure. Roscoe 6 carried out the first reliable determinations of the atomic

weight of vanadium. He used three methods : trioxide (a) Reduction of vanadium pentoxide to vanadium by

of the ratio : the atomic means hydrogen. From V 2O5 V 2O 3 , weight of vanadium was found to be 51-382. 7 (b) Vanadium oxytrichloride was treated with silver nitrate solution volumetrically until all the chlorine was removed. From the ratio VOC1 3 : 3Ag, the atomic weight of vanadium was 51*055. (c) Vanadium oxytrichloride was treated with silver nitrate, and the precipitated silver chloride was collected, dried, and weighed. From

: 51*259. the ratio VOC1 3 3AgCl, the atomic weight of vanadium was No further investigations were carried out until 1909, over forty 8 years later, when Prandtl and Bleyer repeated Methods (a) and (c) of Roscoe, but took care to avoid several sources of error. Method (a) gave the value 51-356, but the investigators found that the vanadium 1 Marino, loc. cit. Berzelius, Pogg. Annahn, 1831, 22, U, Roscoe, J. Chem. 800., 1868, 21, 322; Phil Trans., 1868, 158, 1. Rammelsberg, Pogg. Annahn, 1856, 98, 249. See also Cteudnowitz, ibid., 1863, 120, 17. Roscoe, loc. cit. 7 The following atomic weight values have been used in calculating the atomic weights

in this section : 0-16-000; 01-35457; Ag=107-880; Na= Where necessary, the atomic weights have been recalculated from the original experimental data, using the above values. 8 Prandtl and Zeitsch. Bleyer, anorg, Chem., 1909, 65, 152 ; 1910, 67, 257. VANADIUM AND ITS ALLOYS. 25 trioxide obtained on reduction took up oxygen again so readily that its weight could not be obtained with certainty. The value obtained was therefore too high. Method (c) gave the value 51-074. x In 1914 Briscoe and Little again analysed vanadium oxytrichloride, using Methods (b) and (c). The ratio VOC1 3 : 3Ag was studied in detail of and gave an atomic weight 50-950 ; two measurements of the ratio VOC1 3 : SAgCl gave an atomic weight of 50-952. The investigators ' state, in reviewing the possible sources of error, that the figures are likely to prove a trifle low. Hence the atomic weight of vanadium lies between 50-95 and 50-96, and the higher number is most probably the more correct. This value is in good agreement with a determination 2 carried out in 1910 by McAdam, who employed a totally different reaction. A known weight of anhydrous sodium metavanadate was in a stream of chloride heated hydrogen gas and chlorine ; the residual sodium chloride was fused and weighed. From the ratio NaVO 3 : NaCl an atomic weight of 50-960 was obtained. The values for the atomic weight of vanadium as determined by various investigators since 1863 are summarised in the following table : ATOMIC WEIGHT OF VANADIUM.

The International Committee on Atomic Weights adopted the value 51-0 in 1911. 8 This was altered to 50-96 in 1925 and to 50-95 in 1929.4 The atomic number of vanadium is 23. Examination by the method 5 of mass-spectra has shown that vanadium has no isotopes. Uses of Vanadium, (a) Vanadium Steels. By far the largest proportion of the world's production of vanadium is absorbed in the production of ferrovanadium alloy for the manufacture of vanadium steels, which usually contain up to 0-3 per cent, of vanadium. The effect of the addition of vanadium to a steel is to increase its tensile strength enormously, also its hardness, and its resistance to shock and 6 fatigue. A good carbon steel containing about 1-10 per cent, of carbon has an elastic limit of about 30 tons per square inch and an ultimate 1 Briscoe and Little, J. Ohem. 8oc., 1914, 105, 1310. 2 3 McAdam, J. Amer. Ghem. Soc., 1910, 32, 1603. J. Chem. Soc., 1911, 99, 1867. * 6 Phil 385. Ibid., 1925, 127, 913 ; 1929, 218. Aston, Mag., 1924, 47, 6 Gufflet, J. Iron Steel Inst., 1905, 68, 118 ; McWiUiam and Barnes, ibid., 1911, Iron and Steel 83, 294 ; 1915, 91, 125 ; Portevin, Carnegie Scholarship Memoirs, Institute, 1909, I, 230. 26 VANADIUM, NIOBIUM, AND TANTALUM.

stress of about 60 tons per square inch. Addition of 03 per cent, of vanadium to such a steel raises the elastic limit to 43 tons and the maximum stress to 76 tons per square inch, while addition of 0-6 per cent, of vanadium gives an clastic limit of 65 tons and a maximum 1 stress of 85 tons per square inch. Similar improvement in physical of small of van- properties takes place with the incorporation quantities adium into cast iron. 2 Vanadium steels are admirably suited to situations are where they are subject to varying stresses and vibrations. They used in the construction of transmission shafts, piston rods, axles, bolts, and in tools for shear- gears, motor-car parts, aeroplane parts, punching, coefficients of thermal of vanadium ing, and drawing. The expansion 3 steels are given by Souder and Hidnert. cent, It is found that the use of vanadium up to about 1 per with manufacture of tools tungsten and molybdenum in the high-speed increased again exerts a favourable influence by imparting cutting conditions. The of a small efficiency under heavy working presence the of or proportion of vanadium reduces quantity tungsten molyb- hardness and denum required to impart to the steel definite toughness. in nickel Used in conjunction with other alloying metals, e.g. steels, it cobalt steels, chromium steels, and nickel-chromium steels, produces 4 resemble the chrome- equally useful results. These special steels closely surface nickel steels, but have the advantage of greater freedom from in automobile construction. imperfections. They are employed mainly Some chromium-vanadium steel which has high resistance imparted by heat treatment is used for armour plate of medium thickness, gun use of vanadium steel for shields, gun tubes, torpedo tubes, etc. The 5 the manufacture of Brinell balls has recently been suggested. The function of vanadium in steel appears to be twofold : (a) It acts as a ''scavenger," removing traces of oxygen and nitrogen, and a more distribution of carbon (b) it effects homogeneous throughout the mass. The vanadium displaces the iron from the iron carbide, V C which does not vanadium , Fe 3C, in the steel, producing carbide, 4 3 cementite or with the that segregate as readily as pearlite, consequence the carbon is more evenly distributed throughout the mass and a fine- 6 cast vanadium also grained structure results. In the case of iron, 7 assists the retention of the carbon in the combined form. Vanadium finds application to a limited extent in the manufacture 0-5 of non-ferrous alloys. The introduction of the metal up to about 60 to 70 cent, of per cent, into brasses which contain from per copper and 30 to 40 per cent, of zinc is stated to increase the maximum stress 8 and elongation. Copper-vanadium and aluminium-vanadium alloys

1 442 J. Sac. Chem. 1183. Arnold, J. Iron Steel Inst., 1915, 91, ; Smith, Ind., 1901, 20, a Norris, Met. Chem. Eng., 1911, 9, 361. 3 Souder and Hidnert, American Bureau of Standards, Scientific Papers, 1921, No. 433 ; see also Maurer and Schilling, Stahl und Eisen, 1925, 45, 1160. 4 loc. cit. Guillet, J. Iron Steel Inst., 1906, 70, 71 ; McWiUiam and Barnes, ; Portevin, Soc. Chem. Met. 1081 ; Proc. Amer. Testing Materials, lac cit. ; McAdam, Eng., 1921, 25, for Soc, Steel 393 1923,56; 1924,273,574; Trans. Amer. for Treating, 1924, 6, ; 1925,7,54, Gillett and Proc. Amer. Soc. Materials, 1924, 476, 609, 760. 217, 581 ; Mack, for Testing 6 Quick and Jordan, Trans. Amer. Soc. for Steel Treating, 1927, 12, 3. o and Arnold and Reed, J. Iron Steel Inst., 1912, 85, 215 ; Grossmann Bain, ibid., StaU und 1629. 1924, no, 263 ; compare Maurer, Msen, 1925, 45, 7 318. Donaldson, /. Iron Steel Inst., 1918, 98, 497 ; Hatfield, ibid., 1911, 83, s and /. Inst. Morris, J. Iranklin Inst., 1911, 171, 561 ; but compare Dunn Hudson, Metals, 1914, ir, 152,- Clark, J. Soc. Chem. Ind., Abs., 1915, 34, 1097. VANADIUM AM) ITS ALLOYS. 27 are used in construction an aeroplane ; aluminium-manganese-vanadium alloy is used for castings, and a copper-vanadium bronze is used in 1 marine work. Addition of vanadium also improves the tensile strength and the elastic limit of copper-aluminium bronzes and of copper-zinc- aluminium bronzes, but not beyond the amount due to its deoxidising properties. (b) Vanadium Catalysts. Vanadium compounds function as extremely efficient catalysts in various oxidative reactions of technical importance. They are used in the oxidation of aniline to aniline-black, 2 3 of naphthalene to phthalic anhydride, of anthracene to anthraquinone, 4 of benzene to maleic acid, of toluene to benzaldehyde and benzoic 5 acid, as well as in the conversion of acetylene into acetaldehyde. Among important inorganic processes in which vanadium catalysts are 6 employed are the oxidation of ammonia to nitric acid and of sulphur 7 dioxide to sulphur trioxide. To prepare the catalyst for these operations, pure alumina, or other inert, porous material of the type of pumice or kieselguhr, may be mixed with about 10 per cent, of ammonium metavanadate or other vanadium salt and compressed into briquettes, are the which then strongly heated ; vanadate decomposes and leaves 8 a porous block impregnated with finely divided vanadium pentoxide. Alternatively, ferric chloride, or a soluble salt of another metal, is added to a solution of ammonium metavanadate which has been acidified the is with hydrochloric acid ; precipitated vanadate washed, dried, 9 and gently ignited. Recently, complex vanadium silicates have been introduced, particularly to replace platinum in the contact process for the manufacture of sulphuric acid. It is claimed for them that they are closely parallel to the best platinum catalysts with respect to optimum temperature, conversion equilibrium, and activity, and have the advantages of being cheaper and of not becoming poisoned by 10 arsenic or hydrochloric acid. In the laboratory the vanadium catalyst can be prepared by soaking asbestos fibre in a dilute solution of vanadyl is obtained a solution of sulphate, VOSO 4, which by reducing boiling at to C. ammonium vanadate with sulphur dioxide 40 50 ; the solution is made alkaline with ammonia and evaporated almost to to 500 C. to dryness ; the fibre is then dried and heated about decompose ammonium salts.11 Vanadium salts have also been found to be effective as catalysts for in various electrolytic oxidation and reduction processes ; example 12 in the preparation of hypochlorites, the reduction of polynitro-aromatie 14 compounds, 13 and in the sulphonation of aromatic compounds. On

1 See Amor. Chem. Abs., 1921, 15, 3603. 2 Guyard, Bull Soc. Chim., 1876, [li], 25, 58, 351. 3 Gibbs, J. 2nd. Eng. Chem., 1919, u, 1031. Weiss and Downs, ibid., 1920, 12, 228. Pat. 238033 J. Soc. Chem. See Chem. Trade J. 9 1922, 71, 418 ; Eng. (1924) ; Maxted, Ind 1928, 47, 101. Neumann and Rose, Zeitsch. angew. Chem., 1920, 33, 41, 45, 51. Miles, Manufacture of Sulphuric Acid (Contact Process), vol. iv. (London, 1925), p. 120. UJ5. Pat. 1518043 Dyson, Chem. Age, Metallurgical Section, 1926, 14, 33 ; (1925). Eng. Pat. 15174 (1913). 10 Nickell, Chem. Met. Eng., 1928, 35, 153. 11 Efremov and Rosenberg, J. Russ. Phys. Chem. Soc., 1927, 59, 701. 12 Luther, Zeitsch. Elektrochem., 1907, 13, 437 ; Allmand, Applied Electrochemistry (London, 1924), p. 370. 13 u Hofer and Jacob, Ber., 1908, 41, 3187. Meyer, Annalen, 1923, 433, 327. 28 VANADIUM, NIOBIUM, AND TANTALUM. being incorporated into a drying oil in the form of a rosinate or linoleate, vanadium accelerates oxidation of the oil even more efficiently than lead or manganese, which are commonly used for the purpose, but not 1 so efficiently as cobalt. The oxidised film is smooth and tough. Vanadium cannot, however, be incorporated into oils to be used for 2 white paints, as the film possesses a brown colour. (c) Various Uses. On being fired at a red heat with pottery or glass, vanadium compounds impart a fine gold colour with a greenish, tinge. Vanadium chlorides and other vanadium compounds are used with in the colour potassium ferricyanide toning bromide prints ; green which is produced is attributed to the deposition of yellowish-green 3 vanadium ferrocyanide together with Prussian blue. A solution of vanadium pentoxide in sulphuric acid is used under the name of Mandeliri's Reagent in testing for the presence of various alkaloids. Vanadium Alloys. Vanadium alloys readily with many metals, including aluminium, cobalt, copper, iron, manganese, molybdenum, nickel, platinum, and tin, also with silicon. These alloys have hitherto received scant attention, and little is known in most cases of the systems produced. Aluminium appears to alloy with vanadium in all proportions. The alloys can be prepared by melting aluminium in a crucible and igniting a mixture of vanadium pentoxide on the top, or by reducing a mixture of aluminium, alumina, and vanadium pentoxide, with addi- 4 tion of cryolite and fluorspar, in the electric furnace. They can also be produced by reducing vanadium pentoxide with carbon in the electric furnace in the presence of aluminium, but the product then contains appreciable quantities of carbide. Vanadium-aluminium alloys containing from 30 to 80 per cent, of vanadium have been obtained first-mentioned fusion of these with alu- by the processes ; minium gives products of lower vanadium content. By the regulated action of acids on these alloys crystals having the composition A1 3V and A1V have been isolated, and the existence of a third compound, A1V has been 5 of 2 , suggested. The mechanical properties vanadium- aluminium alloys have not been fully studied, but it has been shown that 2 per cent, of vanadium in aluminium results in substantial increase in the strength and hardness of the rolled and annealed metal. The elongation of the annealed alloy falls, but even with 4 per cent, 6 vanadium it is sufficiently high for most purposes. Aluminium-Silicon. Vanadium possesses the property in common with a large number of other metals of forming complex alloys with aluminium and silicon. 7 Several of these vanadium-aluminium- silicides, each possessing different crystalline form, have been obtained

1 Rhodes and J. Ind. Chen, Eng. Chem., 1922, 14, 222 ; compare Hebler, Farben- Zeitung, 1927, 32, 2077. 2 Farben- Assoc. Swehten, Zeitung, 1927, 32, 1138 ; Gardner, Paint Manufacturers" of U.S., Circular 149 (1922). 3 Photography, Its Principles and Practice, Neblette, p. 470 (Chapman & Hall, London), 1927. 4 J. tSoc. Chem. Clark, Ind., Abs., 1915, 34, 1097 ; Czako, Gompt. rend., 1913, 156, 140 on ; Monograph Vanadium Ores, Imperial Institute, London, 1924, p. 9 ; Moissan, Compt. rend., 1896, 122, 1297. 5 and loc. cit. Matignon Monnet, Gompt. rend., 1902, 134, 542 ; Czako, 6 Stohl nnd 726. Schirmeister, JSisen, 1915, 25, 650 ; Keeney, Mineral Industry, 1917, 7 Vigouroux, Compt. rend., 1905, 141, 951. VANADIUM AND IT8 ALLOYS. 29

by fusing together potassium silicofluoride, aluminium, and ammonium metavanadate in varying proportions. One of the products formed in the presence of a large excess of silicon has the composition VgAlgSi^. 1 It yields hexagonal, prismatic needles of density 4-3 and hardness 5. It is a remarkably stable compound, being unattacked by boiling concentrated hydrochloric acid, sulphuric acid, nitric acid, aqua-regia, or fused potassium chlorate. It reacts, however, with fused alkalis and with hydrofluoric acid to give, in the absence of air, a solution of hypovanadous fluoride, VF 2 , which rapidly oxidises to vanadous fluoride, VF 3 . Compounds of vanadium and silicon are described on p. 107. Copper. Alloys of copper and vanadium are prepared by firing a mixture of vanadium pentoxide, copper oxide, aluminium shot, lime, soda-ash, and fluorspar with the aid of sodium peroxide in a magnesia- 2 lined crucible. Electrolytic methods have also been employed, and are 3 applicable for the preparation of other vanadium alloys. An alloy containing 3-38 per cent, of vanadium and 96-52 per cent, of copper was found to be harder than copper and could be drawn into wire. 4 An aluminium-copper-vanadium alloy has been prepared. Iron. The industrial preparation and uses of iron-vanadium alloys have already been described (see pp. 14 and 25). A laboratory method of preparation consists in passing a mixture of hydrogen and vanadium tetrachloride vapour over iron at 900 C. 5 Alloys of iron and vanadium which contained varying quantities of carbon were 6 prepared by Moissan. The freezing-point curve for alloys of iron and vanadium falls from the melting-point of iron (1530 C.) to a minimum at 32 per cent, of vanadium, and then rises to the melting-point of 7 vanadium (1750 C.). As the alloys are homogeneous the metals form a complete series of mixed crystals. A commercially useful alloy containing about 30 per cent, of vanadium is hard but not brittle, and is is its difficult to pulverise ; its specific gravity about 7-3 and melting- point lies between 1340 and 1400 C. The presence of silicon in the alloy increases its hardness and brittleness. The temper-colours of 8 iron-vanadium alloys have been studied by Tammann and Siebel. Mercury. The solubility of vanadium in mercury is too small for measurement.9 Nickel. Vanadium and nickel are miscible in all proportions in the liquid state up to 36 per cent, vanadium. The solid alloys, which contain up to 20 per cent, vanadium, appear to be homogeneous, but 10 those richer in vanadium consist of two kinds of crystals. These alloys are made by reducing a mixture of vanadium pentoxide and nickel oxide. 11 12 Silver. Vanadium does not alloy with silver. 1 Manchot and Fischer, Annalen, 1907, 357, 129. 2 3 Keeney, loc. cit. Gin, CJiem. News, 1903, 88, 38. 4 Helouis, Kevue de Metallurgie, 1905, 2, 588. 5 Parravo and Mazzetti, JKec. Trav, chim., 1923, 42, 821. 6 Moissan, loc. cit. 7 van Rec. Trav. Vogel and Tammann, Zeitsch. anorg. Chem., 1908, 58, 73 ; Liempt, chim. 1926, 45, 203. 8 Tammann and Siebel, Zeitsch. anorg. Chem., 1925, 148, 297. 9 Tammann and Hinnuber, ibid., 1927, 160, 249. " Giebelhausen, #wZ., 1915, 91, 251. 11 Herrenschmidt, Compt. rend., 1904, 139, 635. 12 Giebelhausen, loc. cit. CHAPTER III. COMPOUNDS OF VANADIUM.

GENERAL PROPERTIES or VANADIUM COMPOUNDS.!

THE compounds of vanadium are numerous and to some extent this is due to the variable of the element. It complicated ; valency

series of oxides : and the oxide forms a VO, V 2 3 , V0 2 , V 2 6 , although it V 2O 7 has not hitherto been isolated, pervanadates derived from are well defined. A monoxide having the formula V 2 has also been reported, but its existence is doubtful; compounds containing niono- valent vanadium are unknown. As is usual in the case of any one element, the acidity of the oxide increases with increasing oxygen content, and basic properties gradually become less marked. Hypovanadous Oxide, VO, was originally mistaken by Berzelius for the element. As in the case of uranium it is difficult to separate vanadium from the last remaining oxygen atom, and this oxide is found to enter as the vanadyl radical into a large number of compounds. Hypovanadous salts can be isolated in the pure state only with difficulty the readiness oxidation are because of with which they undergo ; they among the most powerful inorganic reducing agents known, and frequently evolve hydrogen from aqueous or acid solution. They are best prepared by electrolytic reduction in an inert atmosphere of more highly oxidised compounds. They arc isomorphous with magnesium salts and with the divalent salts of iron, chromium, and manganese. Double sulphates are as of the type VS0 4 .MgSO 4.7H 2O and VSO 4.K 2SO 4.6H 2O known, well as a complex cyanide, K 4[V(CN) G ].3H 20. is in salts Vanadous Oxide, V 2O 3 , almost insoluble acids, but containing trivalent vanadium can be prepared by reducing the tetra- or penta-valent compounds. Vanadous salts are also strong reducing for metallic agents. Vanadous trichloride, VC1 3 , instance, precipitates silver from a solution of a silver salt, the charge on the Ag* ion being taken up by he V*" ion. Vanadous salts are comparable in their salts general behaviour with trivalent iron, aluminium, and chromium ;

form alums with the alkali . they sulphates, e.g. V 2 (S0 4 ) 3.(NH 4 ) 2SO 4

24H as well as double oxalates of the C .V . 2O, type 3(NH 4 ) 2 2 4 2 (C 2 4 ) 3 6H 2O. The trivalent vanadium ion, V*", as in the case of the ferric ion, Fe'", has a strong tendency to produce complex ions, and hence the existence as of such compounds potassium vanadicyanide, K 3[V(CN) 6 ]. is dissolves both in Hypovanadic Oxide, VO 2 , amphoteric, and alkalis and acids. In passing from trivalent to tetravalent vanadium, however, the basic character of the oxide becomes less pronounced and

1 The general characteristics of vanadium compounds are outlined in Chapter I. 30 COMPOUNDS OF VANADIUM. 31

weakly acid properties become manifest. The corresponding hydroxide. has not been isolated it to V(OH) 4 , definitely ; appears be too weakly basic to form salts by the replacement of the four hydroxyl groups by acid the salts radicals, and produced possess the general formula VOX 2 . are VOSO and Examples vanadyl sulphate, 4, vanadyl dichloride, VOC1 2 , in which two hydroxyl groups only have undergone replacement. Vanadium and vanadium tetrafluoride, VF 4, tetrachloride, VC1 4 , however, are known. Considerable numbers of double vanadyl salts are also known. When solutions of hypovanadic oxide in hot alkalis are cooled, salts which contain the vanadium in the anion are obtained. These are called hypovanadates or vanadites, and are probably derived from an acid having the composition 4V0 2.H 2 or H 2V4O9 . Hypovanadic oxide appears to be too weakly basic to give rise to salts without previously

salts . undergoing condensation, then giving of the type R 2O.tcVO 2 Vanadic Oxide or vanadium pentoxide gives rise to the vanadates, the most numerous and important class of vanadium compounds. All vanadium compounds in a lower stage of oxidation have a tendency on being warmed to become converted into vanadates or derivatives of vanadates. Vanadium pentoxide hence is strongly acidic, but the basic character exhibited by the lower oxides is not completely lost, since the pentoxide dissolves in strong acids to give rise to salts, such as, for example, V 2 5 .2SO 3 . These salts, however, are not very well defined or stable, and readily undergo hydrolysis to give rise to compounds which contain the trivalent radical [VO]"*, e.g. vanadium oxy- VOC1 or the monovalent radical for trichloride, 3 , [VO 2]% example, ammonium vanadium dioxyfiuoride, 3NH 4F.VO 2F. The pentavalent ion V appears to be incapable of free existence, since no pentavalent compounds are known, excepting those with oxygen and sulphur, in which all the five valencies are saturated with negative elements or As in the case of the lower vanadium has a groups. oxide, VO 2 , pentoxide strong tendency to form condensed poly-acids which give rise to salts of

the . is type R 2O.ci?V 2O5 In this respect vanadium pentoxide analogous

with other weak acids which are formed from metallic elements ; com- x 2 pare, for instance, tungstic acid and chromic acid, several molecules of which frequently combine with one molecule of a base to form a salt. The chromates and vanadates are in fact so comparable in their general behaviour that the formula V0 3 was at one time assigned to vanadium pentoxide in harmony with Cr0 3 for chromic anhydride. Vanadium pentoxide has the further well-pronounced property of combining with other acid oxides to form heteropoly-acids. The most common acid oxides are phosphorus pentoxide, arsenic pentoxide, molybdenum trioxide, tungsten trioxide, silica. The heteropoly-acids yield well-defined, crystallisable salts with basic oxides. Heats of Formation of the Oxides of Vanadium. The difficulty that is experienced in reducing any of the oxides of vanadium to the metal is attributed partially to their very strongly exothermic nature. 3 Ruff and Friedrich obtained the following figures from combustions

carried out in a bomb calorimeter : calories. (i) 2V+5(i-O 2 )==V 2 5 +437,000 7,000 O (ii) 2V+3(|O 2 ) =V 2 3 +3Q2,OQO =blO,000

1 See this series, Vol. VII., Part III. (1926), p. 208. 2 Ibid., p. 44. 8 Ruff and Friedricli, Zeitsch. anorg. Chem., 1914, 89, 279. 32 VANADIUM, NIOBIUM, AND TANTALUM.

Previous determinations of the heat of formation of the pentoxide gave 313,030 calories l and 250,315 calories. 2 3 Mixter investigated the thermal changes that ensue when the lower oxides are converted into the pentoxide, and from these data calculated the following :

calories. (i) 2V+5(i0 2 )=V a 6 +441,000 (ii) 2V+20 2 =2VO 2 +412,800 (iii) 2V+3(|0 2 )=V 2 3 +353,200 (iv) 2V+0 2 =2VO +208,600

The last figure is in fair agreement with an indirect determination 4 effected by Slade and Higson, who obtained

2V+O 2 =2VO +222,000 calories. It is of interest to note that, commencing with vanadium pentoxide, the amounts of heat absorbed in the successive formation of the next lower oxide rapidly increase as the metal is reached. The heat changes in the following equations have been calculated from Mixter's figures, given above : V =V - calories, (i) 2 5 ~-i0 2 2 4 - 28,200 (ii) V 2 4 -J0 2 =V 2 3 59,600 (iii) V 2 3 -i0 2 =2VO -144,600 (iv) 2VO -0 2 =2V -208,600 The heats of formation of a few other oxides are here inserted for the sake of comparison :

(i) 2P +5(-|0 2 )=P 2 5 +365,300 calories. (ii) 2Fe+3(|0 2 )=Fe 2O 3 +195,600 (iii) 2Cr+3(i-0 2 )=Cr 2 3 +267,000 (iv) 2A1+3(|O 2 )=A1 2 3 +392,600

The comparatively high heat of formation of vanadium pentoxide and the tendency of aluminium to alloy with metallic vanadium explain the non-success of the application of the thermite process for the production of pure vanadium from the pentoxide and from vanadates. The table on the next page summarises the various types of vanadium compounds known. Vanadyl salts are salts of tetravalent vanadium, and contain the divalent [VO]" radical. Many vanadium compounds are known which appear to contain a [VO] group, but the vanadium is either trivalent or pentavalent. Throughout this book the term vanadyl is restricted to compounds of tetravalent vanadium, that is, to salts of the oxide V0 2 . for the which contains Hence, example, compound VOC1 3 , pentavalent vanadium, is called vanadium and not the more usual " o%ytrichloride, by but less logical name vanadyl chloride." 5 Colours of Vanadium Salts. As is usual with salts of metals which exhibit variable valency, those of vanadium are coloured in solution. The colour varies with the salts valency ; of V 2O 5 are yellow, those of

1 Muthmann, Weiss, and Riedelbauch, Annakn, 1907, 355, 58. 2 Buff and Martin, Zeitsch. angew. Chem., 1912, 25, 49. 3 J. Mixter, Amer. Sci, 9 1912, [iv], 34, 141. 4 Slade

technical Catalytic Activity of Vanadium Compounds. The application of vanadium compounds as catalysts has already been referred the velocities of which are affected to (see p. 27). Several reactions, been investi- by the presence of vanadium salts, have quantitatively 3 that the gated. It appears to be established compounds employed and that their effect usually function as oxygen carriers, depends, and reduction. therefore, on the ease with which they undergo oxidation

: reduction of chloric HC10 To give two instances (a) The acid, 3> by the addition of a vanadous hydriodic acid, HI, is accelerated by salt, because chloric acid is much more rapidly reduced by a vanadous salt acid on the other hand, the reduction of than by hydriodic ; persulphuric with HI is not affected addition of a acid, H 2S 2 8 , appreciably by reduces acid but vanadous salt, because the last named persulphuric accelerates the oxidation of sucrose slowly, (b) Vanadium pentoxide and to oxalic acid by nitric acid, that of ethyl alcohol to acetaldehyde

1 175. Fischer, Trans. Amer. Ekctrockem. Soc., 1916, 30, 2 Someya, Zeitsch. anorg. Chem., 1927, 161, 46. s Ostwald, Zeitsch. physiM. Chem., 1888, 2, 127. * Not hitherto isolated. 3 VOL. vi. : in. 34 VANADIUM, NIOBIUM, AND TANTALUM. acetic acid by air, that of potassium iodide to iodine by hydrogen peroxide, and that of stannous salts by nitric acid. In these reactions the vanadium pentoxide gives up its oxygen to the oxidisable substance, 1 being itself reformed at the expense of the oxidising agent. In some cases, however, the modus operandi is modified. In the oxidation of hydriodic .acid with chromic acid, the data indicate that while liberation of iodine takes place, the vanadous or hypovanadic 2 salt employed as the catalyst also undergoes oxidation to vanadate. The vanadium compound here belongs to the class of catalysts known as inductors, and the reaction is comparable to the oxidation in aqueous solution of sodium sulphite with sodium arsenite, whereby both sodium sulphate and sodium arsenate are produced. More recently the conversion of benzene into maleic acid in the presence of vanadium oxides as catalysts has been studied with a view to on the mechanism of such oxidations. The data throwing light " " obtained seem to show that the action depends on an oscillation between the two oxides dissociation of the former V 2O 6 and V 2O 4 , the activated for it is also shown that supplying oxygen the reaction ; but the nature of the products of the oxidation is a function of some other 3 property of the catalyst not yet clearly understood. The presence of a vanadium salt in dilute sulphuric acid solution has also been found to improve the catalytic action of platinum in the 4 combination of hydrogen and oxygen.

The Electromotive Behaviour of Vanadium.

Vanadium precipitates the metal from solutions of salts of gold, silver, platinum, and iridium, and reduces solutions of mercuric chloride, cupric chloride and ferric chloride to mercurous chloride, cuprous chloride, and ferrous chloride, respectively. In these reactions the vanadium passes into solution as the tetravalent ion. No precipitation or reduction ensues, however, when vanadium is added to solutions of divalent salts of zinc, cadmium, nickel, and lead. From reactions it these has been estimated that the electrolytic potential of the change, vanadium (metal) > tetravalent ions, is about 0-3 to 0-4* volt, which is approximately equal to the electrolytic solution pressure of copper. This figure is a little uncertain through the difficulty of securing pure vanadium. 5 When an electrolyte which is without action on vanadium at ordinary temperatures (for example, dilute solutions of mineral acids, of oxalic or acid, of potassium halides) is electrolysed with a vanadium a anode, complex tetravalent vanadium ion is produced. Similarly, electrolysis at 100 C. and in molten chlorides of sodium or zinc gives rise to complex tetravalent vanadium ions. The E.M.F. in each case is found to be independent of the nature of the electrolyte. When, however, solutions of caustic soda or of caustic potash are employed, the vanadium dissolves as a pentavalent ion, irrespective of variations 1 Naumann, Moeser, and /. Lindenbaum, prakt. Chem., 1907, [ii], 75, 146 ; compare Sabatier and Ann. Ghim. Mailhe, Phys., 1910, [viii], 20, 340. 2 Luther and Kutter, Zeitsch. anorg. Chem., 1907, 1. 3 54, Weiss, Downs, and J. 2nd. Burns, Eng. Chem., 1923, 15, 965 j Sje.nse.maiL and Nelson, ibid., 1923, I5> 524. * Hofmann and er. Dolde, t 1924, [B], 57, 1969, 5 Marino, Zeitsch. anorg. Chem., 1904, 39, 152. COMPOUNDS OF VANADIUM. 35

in concentration, temperature, or current density. The pentavalent ion is the tetravalent ion is electro-negative ; strongly hydrolytic, and readily gives rise to the vanadyl ion [VO]**, which is weakly electropositive. The trivalent vanadium ion displays more marked electropositive properties, which again increase with the formation of divalent ions. 1 a In 1898, Cowper-Coles claimed to have successfully effected the electrolytic reduction of an acid solution of vanadium pentoxide to metallic vanadium, but the product was subsequently shown by Fischer 3 to have been a deposit of platinum hydride. Fischer, in a series of over three hundred experiments, varied the temperature, current density, cathode material, concentration, electrolyte, addition agent, and construction of cell, but in not one instance was the formation of any metallic vanadium observed. In most cases reduction ceased at the tetravalent state (blue). At temperatures above 90 C. reduction appeared to proceed to the divalent state (lavender). The use of carbon electrodes led to the trivalent state (green), but only lead electrodes produced the trivalent state at temperatures below 90 C. Platinum electrodes reduced the to the blue salt below 90 C. electrolyte vanadyl ; using a divided cell and temperatures above 90 C. the lavender salt was obtained. Electrolytic reduction of pentavalent and tetravalent vanadium salts has frequently been employed for the preparation of vanadium 4 5 compounds of lower valency. Bleecker has also prepared vanadium pentoxide and several vanadates electrolytically. The passivity of vanadium is referred to on p. 23, and the electrolytic decomposition of anhydrous fused vanadium salts on p. 17.

VANADIUM AND HYDROGEN. " " Roscoe, in 1870, found that vanadium absorbs or combines with up to 1-3 per cent, of its weight of hydrogen, according to the state of division of the metal, when heated in a current of the gas, and con- 6 firmed the observation in the following year. Muthmann, Weiss, and Riedelbauch subsequently reported that the amount of hydrogen taken up by the vanadium increases with increase of temperature and duration at C. a stable of of contact ; they stated that 1300 hydride vanadium was produced containing 16-1 per cent, of hydrogen. This is described as a black powder which is unaffected by air, hot water, or boiling hydrochloric acid. 7 Prandtl and Manz, however, were unable to obtain any compound of vanadium and hydrogen, and state that the previously observed increases in weight were due either to (a) absorption of oxygen

1 Rutter lias measured the KM.3T.'s associated with the successive changes, v IV m n the of solutions these V >V ^V *-V , and has examined behaviour containing ions towards numerous oxidising and reducing agents (Zeitsch. anorg. Chem., 1907, 52, 368) 2 198 Chem. Cowper-Coles, Institution of Mining and Metallurgy, 1898-99, p. ; News, 1899, 79, 147. 3 see also Fischer, Trans. Amer. Electrochem. Soc., 1916, 30, 175 ; Gin, ibid., 1909, i<5, 439. 4 Piccini, Zeitsch. anorg. Chem., 1899, 19, 204; Piccini and Marino, ibid., 1902, 32, 55; 394 Stahler and 1906, 50, 49 ; Piccini and Brizzi, ibid., 1896, n, 106 ; 1899, 19, ; Wirthwein, Ber., 1905, 38, 3978; Bultemann, Zeitsch. Ekktrochem., 1904, 10, 141 ; Rutter, ibid., 1906, 250. 12, 230 ; Chapman and Law, Analyst, 1907, 32, * Bleecker, Met. Chem. Eng., 1910, 8, 666 ; 1911, 9,501. 6 Roscoe, J. Chem. Soc., 1870, 23, 358 ; 1871, 24, 23. 7 Muthmann, Weiss, and Riedelbauch, AnnaUn, 1907, 355, 58. 36 VANADIUM, NIOBIUM, AND TANTALUM.

and nitrogen, or (6) absorption of hydrogen by impurities in the van- 1 adium. More recent investigations with vanadium of 90 per cent, purity which had been preheated for one hour in a vacuum at 1100 C. have shown that the quantity of hydrogen absorbed varies with the temperature and the pressure. One gram of the metal absorbs 122*6 c.c. of hydrogen at ordinary temperatures and pressures and 2-01 c.c. at 2 1100 C. It is, therefore, improbable that a definite compound is 3 produced. Vanadium and the Halogens.

The variable valency of vanadium is well displayed in its halides and oxyhalides. These are set out in the following table : HALIDES AND OXYHALIDES OF VANADIUM.

The table shows that the stability of the halides decreases with increase in the atomic weight of the halogen. All the halides are hygroscopic and show a very strong tendency to undergo hydrolysis, a tendency which increases with the valency. The tetrabromide and isolated tetriodide have not been ; VF 4 and VC1 4 can perhaps be regarded as salts of the very weak base V0 a . They are easily fusible compounds, and hydrolysis so readily that they evolve the gaseous undergo " halogen acid and "fume in moist air; they are therefore comparable with the tetrahalides of titanium, germanium, tin and lead. The preparation of the anhydrous halides of vanadium is possible only in the dry way, since attempts to remove the water from the hydrated halide result in the formation of a basic salt, as in the cases of the halides of iron, chromium, aluminium, bismuth, etc. The general methods of preparation employed are :

1 Prandtl and Manz, Zeitsch. anorg. Chem., 1912, 79, 209. 2 Huber, Kirschfeld, and Sieverts, Ber., 1926, 59, 2891. 3 See also Biltz, Zeitsch. anorg. Chem., 1928, 174, 42. * Does not exist in the free state. COMPOUNDS OF VANADIUM. 37

1. Halogenation of the oxides or sulphides. 2. Reduction of a higher halide either directly or by means of hydrogen. 3. Direct synthesis. Since fluorine is difficult to prepare and manipulate, the anhydrous fluorides and oxyfluorides are prepared by the action of anhydrous 1 hydrogen fluoride on other halides or oxyhalides of vanadium.

VANADIUM AND FLUORINE.

Vanadium combines directly with fluorine to produce a mixture of several fluorides, from which it is difficult to separate any in the pure state. vanadium has not been Hypovanadous Fluoride, difluoride, VF 2, isolated, but a reddish-violet solution containing it is obtained by the 2 action of excess of hydrofluoric acid on vanadium aluminium silicide, Si in the absence of air. In the of air the solution V8Al 2 13 , presence becomes green, with evolution of hydrogen :

2VF a +2HF=2VF 3 +H 2 . 3 The solution behaves generally like one of vanadium dichloride. is Vanadous Fluoride, vanadium trifluoride, VF 3 , produced when hydrogen fluoride is allowed to react with pure, dry vanadous chloride, a dark red heat. It is a almost VC1 3 , at greenish-yellow powder, insoluble in water and the usual organic solvents. It sublimes at a at 19 3-362S.4 .3H bright red heat. Density C., The trihydrate, VF 3 20, separates in dark green rhombohedral crystals when vanadous oxide, is in acid and the solution is V 2O 3 , dissolved aqueous hydrofluoric concentrated. This fluoride has also been prepared by the electrolytic 5 reduction of a solution of vanadyl difluoride in hydrofluoric acid. to air It decomposes rapidly, with absorption of oxygen, on exposure or in solution. It reduces silver salts to the metal, and mercuric and With cupric salts to the mercurous and cuprous state, respectively. alkali carbonates and hydroxides it reacts to throw down a volumin- of vanadous which also ous greyish-green precipitate hydroxide, V(OH) 3 , oxidises in the air. 6 Vanadous fluoride possesses the property common to many fluorides 7 of forming a large number of easily crystallisable double salts with the fluorides of other metals and with ammonium fluoride. The following green, crystalline compounds have been pre- 8 pared and described : VF .CoF .7H VF 3.NH 4F.2H 2O VF..KF.2ELO 3 2 2 O VF .NiF .7H O VF 3 .2NH 4F.H 2 VF,.2KF.H3' 9 3 2 2 .RbF.2H VF .ZnF .7H VF 3.3NH 4F VF 3 2 3 2 2 .CdF .7H 2VF,.5NaF.H 2 VF 3.CsF.2H 2 VF 3 2 2 VF 3.T1F.2H 2O VF 3.2T1F.H 2O The constitution of these compounds is discussed later (see p. 39). 2 i Ruff and Lickfetfc, Ber., 1911, 44, 2539. See p. 29. 4 3 Manchot and Fischer, Annalen, 1907, 357, 129. Ruff and Lickfett, loc. cit. 5 Scarliarini and Airoldi, Gazzetta, 1925, 55 44. e 47. Petersen, J. prakt. Chem., 1889, ii], 40, 7 45 et See this series, Vol. VIII. (1915), pp. seq. . 8 18, 186 ; 1892, 22, 55 ; Petersen, ibid. ; Piccini and Giorgis, Gazzetta, 1888, Ephraim J. Chem. Abs., 1910, 98, ii, 619. and Heymann, Ber., 1909, 42, 4460 ; Costacheseu, Soc., 38 VANADIUM, NIOBIUM, AND TANTALUM.

is Vanadium Tetrafluoride, hypovanadic fluoride, VF 4 , produced by the action, of anhydrous hydrogen fluoride on the corresponding chlorine VC1 at from 28 to C. It is compound, 4 , temperatures an extremely deliquescent brown powder which readily undergoes hydrolysis with water, so that it does not give rise to double salts as in the case of the trifluoride. Its density at 23 C. is 2-9749. It de- of the the composes above 325 C., yielding a mixture pentafluoride and 1 trifluoride :

2VF 4 =VF 3 +VF 3 .

is of as Vanadium Pentafluoride, vanadic fluoride, VF5J interest be action the only pentahalide of vanadium. It cannot prepared by the V or on sodium meta- of hydrofluoric acid on vanadium pentoxide, 2 5 , because of its to vanadate, NaV0 3 , very strong tendency undergo It is obtained as a hydrolysis, with the formation of oxyfluorides. white sublimate when the tetrafluoride is heated in a current of nitrogen in a platinum tube, the temperature gradually being raised from 300 in moist air it is therefore to 650 C. It attacks glass and decomposes ; It is soluble in the common kept in air-tight platinum tubes. organic solvents. Density at 19 C., 2-1766. It boils at 111-2 C. under 2 758 mm. pressure. Vanadium Oxyfluorides.

is the action of Vanadyl Difluoride, VOF 2 , prepared by anhydrous VOBr . It is a sub- hydrogen fluoride on vanadyl dibromide, 2 yellow 3 stance. Density at 19 C., 3-3056. A hydrated vanadyl difluoride, blue when VOF 2.#H 20, is obtained as microscopic crystals hypovanadic is in excess of acid and the solution oxide, V0 2 , dissolved hydrofluoric 4 concentrated slowly over sulphuric acid. The following double salts 5 and stable : have been prepared, all of which are crystalline fairly .SNaF.2H VOF .CoF .7H VOF 2.2NH 4F 3VOF 2 2 2 a 2

- VOF .NiF .7H VOF 2.2NH 4F.H 2 VOF..2KF2 2 2 2 .7KF VOF .ZnF .7H VOF 2 .3NH 4F 3VOF 2 2 2 2 VOF .CdF .7H 4VOF 2.7NH 4F.5H 2O VOF..2T1F 2 2 2

is obtained the action of Vanadium Oxytrifluoride, VOF 3 , by

chlorine . hydrogen fluoride on the corresponding compound, VOC1 3 It is also formed when vanadous fluoride is heated to redness in oxygen. It is a yellowish-white, extremely hygroscopic substance/ readily 6 soluble in water. Its density at 20-5 C. is 2-4591. When heated to formation of 132 C. it undergoes partial decomposition with vanadium pentoxide. The following double salts of vanadium oxytrifluoride are 7 VOF .VF . known: 2VOF 3 .3NH 4F.H 2O, 2VOF 3 .3KF, 3.2KF, VOF 3 5 double salts are 4KF, and 2VOF 3.3KF.HF. These white, crystalline are stable in the air, but in compounds ; they undergo hydrolysis of vanadium F. solution to form addition compounds dioxyfluoride, V0 2

1 Ruff and Lickfett, Ber., 1911, 44, 2539. 2 Ruff and Lickfett, loc. tit. Ruff and Lickfett, loc. cit. 4 Petersen, J. prakt. Chem., 1889, [ii], 40, 194. 6 Piccini and loc. cit. Petersen, loc. cit. ; ger., 1888, 21, 3257 ; Giorgis, ; Baker, JBer., 4460. 1878, ii, 1722 ; Ephraim and Heyniann, Ber., 1909, 41, 6 Ruff and Lickfett, loc. cit. 7 Piccini and loc. cit. Petersen, J. yrakt. Chem., 1889, [ii], 40, 271 ; Giorgis, COMPOUNDS OF VANADIUM. 39

Vanadium Dioxyfluoride, V0 2F, has not been isolated in the free state. It combines readily with other fluorides to form a series of yellow, easily crystallisable double salts, which can be heated to 100 C. or even above without undergoing change. The following have been 1 prepared : F.H V0 2F.2NH 4 2 VO 2F.2KF V0 2F.ZnF 2 .7H 2O

V0 2F.3NH 4F 2VO 2F.3KF V0 2F.BaF 2

2V0 2F.3NH 4F 2VOJF.3T1F V0 2F.CaF 2 2V0 2F.3NH 4F.H 2O

Constitution of Vanadium Double Halides.2 The double fluorides of vanadium and other double halogen salts in most cases can be regarded as in accordance with Werner's theory of co-ordinated compounds. The vanadium has a co-ordination number six.

The hydrated fluoride, VF 3 .3H 20., may be regarded as an aquo-salt, "1 The of tFV,TY Q\ gradual replacement aquo-water by fluorine gives 3 the following : V [ (H aO) i.e.

VF 3 .NH 4F.2H 2 ; VF 3 .2NH4F.H 2O ; VF 3.3NH 4F.

the double salt, VF .2KF.H 0, is . The Similarly, potassium 3 2 fv^g- Q, |K 2 alkali metal can be replaced by aniline to produce salts of the same * type:

and [VF6](NH 3C6H5 ) 3.

The constitution of the acid radical of the potassium salt containing one molecule of water corresponds to that of those double salts which contain seven molecules of water. It therefore follows that the acid radical in the latter is also co-ordinatively saturated. The salt

VF .ZnF .7H O thus becomes 6 of 3 2 2 [~V7^ /l[Zn(H 2O) ]. Replacement the zinc atom by atoms of other metals gives the corresponding cobalt, 5 nickel, and cadmium compounds, which have similar constitution. Double salts of the other vanadium halides are much less stable and therefore less numerous than those containing fluorine. They are represented on the co-ordination theory by formulae which are analogous to those set out above. Unlike vanadium trifluoride, which of forms VF 3.3H 20, the molecules of the corresponding hydrates

1 loc. cit. J. Zeitsch. Piccini and Giorgis, ; Baker, Ghem. Soc., 1878, 33, 388 ; Ephraim, loc. cit. and anorg. Chem., 1903, 35, 66 ; Petersen, ; Ephraim Heymann, er., 1909, 41, 4460. 2 For a general discussion of double fluorides and oxyfluorides of the metals, see this series, Vol. VIII. (1915), pp. 45 et seq. 3 Werner, New Ideas in Inorganic Chemistry, translated by Hedley (Longman's, 1911), p, 120. * Costachescu, J. Chem. Soc., Abs., 1910, 98, ii, 619. 5 Werner, loc. cit. 9 137. 40 VANADIUM, NIOBIUM, AJSTD TANTALUM. the other are halogen compounds found to possess 6H 20, and are written :

[V(H 20)6]C1 3 ; [V(H,0) 6]Br 3 ; [V(H 20) 6]I 3 .

1 It is of interest to note that Meyer and Backa, by treating vanadium trichloride and tribromide with liquid ammonia, have recently obtained hexammine derivatives :

[V(NH 3 )6]C1 3 ; [V(NH 3 ) 6]Br 3 , These are comparable in their reactions to the hexammine of ferric . its chloride, [Fe(NH 3 ) 6]Cl 3 Hexammino-vanadium trichloride loses chlorine on being treated with nitric acid and forms the corresponding nitrate, [V(NH 3 ) 6 ](N0 3 ) 3 . The double oxy-salts may be regarded as belonging to the following general series :

VOF .2NH F.H can be written . Thus, a 4 2 VO ) 2 ; VOF 2 .ZnF 2 | |(NH 4

4 ~1 VO [Zn(H,0)J; V0 2F.3NH 4F as [FiHaOj F f 3 1 and F.ZnF .7H as It will VO 2 a aO V0 2 [Zn(H 20)6]. be observed I TT (~\ I I | XlftV/

that the co-ordination number six is maintained by bringing a molecule of water into the co-ordinated complex. Ephraim has observed that the composition of the halogen double salts of vanadium and other metals appears to be dependent on the atomic weight of the second metal This is shown by rewriting the formulae

for some of salts : the double 2VF 3.6NH 4F, 2VF 3 .5NaF, and 2VF 8.4KF. It has not been found possible to prepare double salts in which the number of molecules of alkali fluoride combined with two molecules of is VF 3 greater than 6, 5, and 4 respectively. Efforts to prepare a double salt having the composition 2VF 3.5KF, for instance, were unsuccessful. Similarly, rearranging the formula for some double salts of 2 vanadyl fluoride gives: 3VOF 2 .9NH4F, 3VOF 2 .8NaF, and

VANADIUM AND CHLORINE.

vanadium VC1 . Solutions Hypoyanadous Chloride, dichloride, 2 of vanadium dichloride can be prepared by electrolytic reduction of 3 higher chlorides, or by the addition of amalgamated zinc to a hydro- 4 chloric acid solution of vanadium pentoxide. The solution undergoes very rapid oxidation, hence the isolation of vanadium dichloride cannot

1 Meyer and Backa, Zeitech. anorg. Chem., 1924, 135, 177. 8 Ber. Ephraim, t 1903, 36, 1177 ; compare Grossinann, ibid., 1903, 36, 1600 ; Melikoff and Kasanezky, J. Ru$$. Phys. Chem. Soc., 1904, 36, 77. 8 Piccini and Marino, Zeitsch, anorg. QJiem., 1902, 32, 67. * Conant and Cutter, J. Awer. Ch&m. Soc., 1926, 48, 1023. COMPOUNDS OF VANADIUM. 41 be effected a wet method. It is by deposited in apple-green hexagonal flakes when the of vapour vanadium tetrachloride mixed with* dry is hydrogen passed through a glass tube heated to dull redness * Vanadium trichloride undergoes decomposition to the dichloride and tetrachloride when heated to 900 C. in a stream of nitrogen or carbon dioxide. The tetrachloride produced at the same time distils away at the of the reaction. 2 temperature More recently, vanadium dichloride has been obtained by the action of hydrogen chloride on commercial ferrovanadium. 3 Its density at 18 C. is 3-23 It is a hygroscopic solid which with deliquesces oxidation to a brown liquid In the of mineral acid a presence violet solution is formed, which like all other solutions of divalent vanadium salts, acts as an extremely powerful it in one reducing agent; is, fact, of the strongest deducing known. It agents slowly evolves hydrogen from its aqueous or acid solutions, with formation of the trichloride a ; vigorous evolution of takes from the acid hydrogen place solution in the presence of platinum toil. It bleaches litmus and indigo, precipitates the metals from solutions of salts of copper, tin, silver, gold and platinum, and has been successfully in the isolation of employed certain organic radicals, as tor the example, tnphenylmethyl radical from triphenyl carbinol 4 At a red heat it reduces carbon 5 bright dioxide to the monoxide :

3VC1 2 VC1 +2CO 3 =2VOC1+ 4 +2CO ; and with even the stronger heating reaction proceeds further, 4VC1 2 +3C0 2 =V 2 3 +2VC1 4 +3CO, pure vanadium sesquioxide being produced.

Vanadous Chloride, vanadium . trichloride, VC1 3 This halide is obtained the action of by hydrogen chloride on finely divided vanadium at 300 to 400 6 C., or by heating vanadium tetrachloride to 140 C, in a current of carbon dioxide, which removes the chlorine formed at the same time. It can be conveniently made also by boiling vanadium oxytrichloride, VOC1 3 , vanadium VC1 or tetrachloride, 4, a mixture of both, with under reflux. 7 sulphur, The reactions involved are : 2VC1 (i) 4 +2S =2VC1 3 +S 2C1 2 .

2VOC1 . (ii) 3 +S =2VC1 3 +S0 2 In the former case the sulphur chloride is distilled off, and in both cases the excess of is removed sulphur by distillation in an atmosphere of carbon dioxide or nitrogen. Anhydrous vanadium trichloride is a crystalline solid of the colour of blossom. It is peach extremely hygroscopic, deliquescing to a brown It liquid. gives green solutions in alcohol and ether. Density at 18 3*00. The 8 C., absorption spectrum has been studied. On being heated in it is reduced to strongly hydrogen vanadium dichloride, VC1 2, * Roscoe, J. CJiem. Soc., 1870, 23, 344. 2 Ruff and Lickfett, Ber. t 1911, 44, 506. 3 Meyer and Backa, Zeitech. anorg. Chem., 1924, 135, 177. 4 Conant and J, Amer. Sloan, Chem. Soc., 1923, 45, 2466 ; Conant and Lutz, ibid., 1047 1923, 45, ; Conant and Cutter, ibid., 1926, 48, 1016. 5 Ruff and Lickfett, loc. cit. 6 Meyer and Backa, loc. cit. 7 Ruff and Liokfett, loc. cit. 8 Morgan and Moss, J. Ghem. Soc., 1913, 103, 88. 42 VANADIUM, NIOBIUM, AND TANTALUM. the dichloride. 1 and ultimately to the metal. Tin also reduces it to heat it is decom- On being heated in a current of nitrogen to a dark red which posed into the tetrachloride, which distils off, and the dichloride, remains behind :

2VC1 3 =VC1 4 +VC1 2 .

The following change also takes place :

for each so that the quantity of tetrachloride produced is determined of the chlorine. If the tri- temperature by the partial pressure chloride is heated in a current of chlorine which is free from oxygen, complete conversion into the tetrachloride ensues. is The VC1 3 .6H 20, prepared by dissolving yanadous kexahydrate, 2 in acid in the absence of air, or by hydroxide, V(OH) 3 , hydrochloric V O the electrolytic reduction of a solution of vanadium pentoxide, 2 5 , 3 in hydrochloric acid. It is a green, crystalline compound, very or ether. On hygroscopic, and readily soluble in water, alcohol, being Its heated it decomposes before all its water has been driven off. concentration ; aqueous solutions are brown or yellow, according to the on addition of acid the solution assumes the green colour characteristic of solutions of trivalent vanadium salts, and as oxidation to the blue. When tetravalent state takes place the solution becomes hydrated vanadium trichloride and rubidium chloride are together solution is dissolved in water, in the requisite proportions, and the of a double saturated with hydrochloric acid and concentrated, crystals The ammonium, salt, VCl 3.2RbCLH 20, are obtained. corresponding 4 The potassium and caesium double salts have been prepared. mag- .H 5 another nesium double chloride has the composition VCl 3 .MgCl 2 2 ; is known. 6 Addition of silver nitrate double potassium salt, VC1 3.KC1, of a mixture of metallic silver and silver gives a precipitate which consists a of thallium chloride ; with thallium sulphate, however, precipitate chloride is thrown down, from which it can be inferred that vanadium in the case of trichloride undergoes normal ionisation in solution, as CrCl and ferric chloride, the corresponding violet chromium chloride, 3 , FeCl 3 .

. The Hevammino-vanadium Trichloride, [V(NH 3 ) 6 ]C1 3 similarity between the trichlorides of iron and vanadium is further shown in their behaviour towards ammonia. At ordinary temperatures ammonia reacts with vanadium trichloride to produce vanadium nitride, VN, and ammonium chloride, but if liquid ammonia is poured over vanadium

a , trichloride, a quantitative yield of reddish-brown salt, [V(NH ) 6]C1 3 is obtained, which recalls the hexammines of cobalt and chromium. The ammonia molecules are not, however, very firmly held, and the with the compound is, therefore, more comparable corresponding

. hexammine of ferric chloride, [Fe(NH 3 ) 6]Cl 3 Hexammino-vanadium trichloride is insoluble in water, alcohol, and

1 Kremann and Noss, Monatsh., 1913, 34, 54. 2 Locke and Edwards, Amer. Ohem. J. t 1898, 20, 594. 3 Piccini and Brizzi, Zeifsch. anorg. Chem., 1899, 19, 394. 4 Stabler, Ber., 1904, 37, 4411. 5 Stabler, loc. cit. 6 Locke and Edwards, Zoo. cit. COMPOIHSTDS OF VANADIUM. 43 ether. On being exposed to dry air it decomposes slowly and becomes white :

[V(NH 3 ) 6 ]C1 3 +0+2H 20=NH 4V0 3 +3NH 4C1+2NH 3 .

On being exposed to damp air it slowly becomes green, due to the formation of the . hexa-aquo-vanadium salt, [V(H 2O) 6]C1 3 It is decomposed by boiling water,

[V(NH 3 ) 6 ]C1 3 +3H 20=V(OH) 3 +3NH 3 +3NH 4C1, and dissolves in dilute hydrochloric acid to give a solution which possesses the green colour characteristic of trivalent vanadium salts. By the action of nitric acid a quantitative yield of hexammino-vanadium is obtained. 1 nitrate, [V(NH 3 ) 6 ](NO 3 ) 3 ,

Hypovanadic Chloride, vanadium tetrachloride, VC1 4 , can be from the lower VC1 in prepared synthetically chloride, 3 , by heating a stream of chlorine at 600 C. Another convenient method consists in passing dry chlorine over ferrovanadium contained in a hard glass tube heated in a combustion furnace. The reaction is expressed :

2FeV+7Cl 2 =2FeCl s +2VCl 4 .

The vanadium tetrachloride distils over and is purified from any ferric chloride present either by distillation or by extraction of the product with carbon tetrachloride, in which only the vanadium halide is soluble. 2 Sulphuryl chloride, thionyl chloride, sulphur monochloride, and phosgene can all be used in the last reaction instead of chlorine, and the ferrovanadium also substituted 3 can be by vanadium carbide, V4C 3 , 4 5 6 or 7 nitride, VN, subsilicide, V 2Si, disilicide, VSi a , pentoxide. Vanadium tetrachloride is a reddish-brown, viscous liquid, which 8 boils at 153-7 C. at 768 mm. and melts at 109 C. Density at C., 9 1-8584. The dielectric constant has been investigated. The electrical 10 conductivity is extremely low even near the boiling-point. On being heated to 900 C. in the presence of iron it undergoes reduction to the 11 metal. It decomposes slowly and spontaneously in a dry atmosphere at into vanadous VC1 and chlorine. ordinary temperatures chloride, 3 , In moist air it undergoes slow hydrolysis with formation of hydro- effect on chloric acid, which gives rise to the usual fuming ; being thrown into water it is immediately hydrolysed and forms vanadyl di a blue solution. The tetrachloride is chloride, VOC1 2 , which gives 12 stable only at temperatures above about '630 C. It is too unstable to form double salts with other chlorides. In benzene solution, however, it reacts with ammonia and its derivatives, e.g. aniline, to yield com-

1 Meyer and Backa, Zeitsch. anorg. Chem., 1924, 135, 177. 2 loc. cit. Mertes, J. Amer. Chem. Soc., 1913, 35, 671 ; Meyer and Backa, 3 Ruff and Lickfett, Ber., 1911, 44, 506. * Roseoe, J. Chem. Soc., 1870, 23, 344. 5 Moissan and Holt, Ann. Chim. Phys., 1902, [vii], 27, 277. 6 Meyer and Backa, loc. cit. 7 Matignon and Bourion, Gompt. rend., 1904, 138, 631. 8 Ruff and Lickfett, loc. cit. ; Biltz and Keunecke (Zeitsch. anorg. Chem., 1925, 147, 171) give the boiling-point as 148*5 0. at 755 mm. 9 Loomis and Schlundt, J. Physical Chem., 1915, 19, 734. 10 Voigt and Biltz, Zeitsch. anorg. Chem., 1924, 133, 277. 11 Parravano and Mazzetti, JRec. Trav. chim., 1923, 42, 821. 12 Ruff and Friedrich, Zeitsch. anorg. Chem., 1914, 89, 279. 44 VANADIUM, NIOBIUM, AND TANTALUM. pounds which consist of one molecule of the tetrachloride in association with from three to six molecules of ammonia or its derivative. 1

Vanadium Oxychlorides.

Divanadyl Chloride, V 2O 2C1, is formed, together with vanadium oxymonochloride and vanadyl dichloride, when the vapour of vanadium is a oxytrichloride, VOC1 3 , mixed with hydrogen, passed through glass tube heated to redness. It is an insoluble, bronze-coloured substance, similar in to the it is seen appearance mosaic gold ; under microscope to consist of glistening yellow crystals. Density at 20 C., 3*64. On 2 being heated in air it forms vanadium pentoxide. A hydrate, 2V 2 2C1.5H 20, has been obtained by the action of hydrogen sulphide 3 on a solution of vanadium pentoxide in concentrated hydrochloric acid. The chemical identity of divanadyl chloride is, however, a matter of doubt. Vanadium Oxymonochloride, VOC1, can be obtained by heating vanadium trichloride in an atmosphere of carbon dioxide at about

700 C. for several hours :

3VC1 3 +2C0 2 =VC1 4 +2VOC1+2CO. It can also be prepared by the reduction of vanadium oxytrichloride, in at a red heat. It is VOC1 3 , hydrogen a flaky, brown, crystalline substance at 16 2-824 soluble in water ; density C., ; with difficulty 4 but readily soluble in nitric acid. is the Vanadyl Dichloride, VOC1 2 , produced by partial hydrolysis of vanadium tetrachloride. It is also formed, together with small proportions of vanadium oxymonochloride, VOC1, when vanadium is reduced either of or oxytrichloride, VOC1 3 , by means hydrogen by 5 heating with metallic zinc in a sealed tube at 400 C. It yields grass- green deliquescent crystals which have an unctuous feel. Density at 6 7 13 C., 2*88. The dielectric constant and absorption spectrum have been investigated. Efforts to obtain double salts of vanadyl dichloride with chlorides of other metals, corresponding to those given by vanadyl difluoride, have not proved successful. series of Two double salts having the general formulae VOC1 2 .4R.HC1. and where or #H 2 VOC1 2 .2R.HC1.#H 2 ? R=quinoline pyridine, are, however, known. 8 Vanadium as Oxytrichloride, VOC1 3 , commonly known "vanadyl chloride," is the easiest to prepare of all the halogen or oxyhalogen compounds of vanadium. It distils over as a yellow liquid when vanadium pentoxide is strongly heated, or when vanadiun trioxide is gently heated, in a current of chlorine. Addition of charcoal accelerates 9 the reactions :

(i) 2V 2 5 +GC1 2 =4VOC1 3 +302 . (ii) 8V 2 3 +6C1 2 =4VOC1 3 +V 2 5 .

1 Mertes and Fleck, J. Ind. Eng. Chem., 1915, 7, 1037. 2 Roseoe, J. Chem. Soc., 1868, 21, 348. 3 Crow, ibid., 1870, 30, 457. 4 Ruff and Lickfett, Ber. 9 1911, 44, 506; Roseoe, kc. cit. 5 Roseoe, loc. cit. 6 Loomis and Schlundt, J. Physical Chem., 1915, 19, 734. 7 Morgan and Moss, J. Chem. $oc., 1913, 103, 88. 8 ICoppel, Goldmann, and Kaufmann, Zeitsch. anorg. Chern,, 1905, 45, 346. 9 J. Roseoe, Chem. Soc., 1868, 21, 342 ; Briscoe and Little, ibid., 1914, 105, 1316. COMPOUNDS OF VANADIUM. 45

Instead of using chlorine, gaseous hydrogen chloride may be employed, provided that a strong dehydrating agent, phosphorus pentoxide or zinc chloride, is present to remove the water formed and so stop the 1 right-to-left reaction in

V 2 5 +6HC12^VOC1 3 +3H 20.

Halogenation in the presence of sulphur, or by means of sulphur halides, is also available. When chlorine is conducted into a mixture of vanadium pentoxide and sulphur, or when powdered vanadium pentoxide is treated with sulphur monochloride vapours, an immediate reaction 2 sets in, with formation of vanadium oxytrichloride. A quantitative yield of this compound is also obtained when vanadium trichloride is heated in oxygen at 500 to 600 C. 3 Vanadium oxytrichloride is a light yellow, mobile, transparent 4 liquid, which boils at 127-15 C at 764-5 mm. Other determinations 5 gave the b.pt. as 126-7 C. at 767 mm. and 124-4 C. at 723 mm. The 6 7 density has been found to be 1-8656 at C., or 1-8362 at 15-5 C., compared with water at 4 C. The expansion on being heated from 8 to 130 C. has been measured. The oxytrichloride remains a liquid at 15 C., and its vapour density at 186 C. is 6-108 (air =1), showing it to be undeeomposed. It is not decomposed on prolonged boiling with metallic sodium, potassium, or magnesium. On exposure to damp air, or on being treated with a small quantity of water, it displays the usual unstable character of the pentavalent vanadium halides in that it undergoes rapid hydrolysis and thereby becomes coated with red flakes of vanadium pentoxide :

2VOC1 3 +3H 2O=V 2O5 +6HC1.

it dilute With more water, passes into solution ; solutions, which are brownish-yellow or red, according to the concentration, evolve chlorine and become blue on standing, undergoing reduction to the tetravalent state. On evaporation to dryness, all the chlorine is evolved and the residue consists of the. pentoxide. Vanadium oxytrichloride is also soluble in ether and alcohol. It dissolves chlorine, bromine, iodine, yellow phosphorus, and sulphur, and is miscible with various liquid its use as an industrial hydrocarbons and chlorinated hydrocarbons ; 9 solvent has therefore been suggested. Addition of pyridine hydrochloride to an alcoholic solution of vanadium oxytrichloride yields an addition compound, VOC^.C^HgN. 10 with a HCLC 2H5OH. On being brought into reaction cupric oxide, is dark green copper chlorovanadate formed, Cu(VO 3 ) 2.CuCl; litharge .PbCl . similarly yields a brick-red lead chlorovanadate, Pb(V0 3 ) 2 2 From the instability of these compounds towards water it is assumed

1 Ephraim, Zeitsch. anorg. Chem., 1903, 35, 66. 2 Matignon and Bourion, Compt. rend., 1904, 138, 631. 3 Buff and Lickfett, loc. cit. * Thorpe, J. Chem. Soc., 1880, 37, 349. 5 loc. cit. see also Prandtl and Bleyer, Zeitsch. anorg. Chem., 1909, 65, 152 ; Roscoe, ; L'Hote, Compt. rend., 1885, 101, 1151. 6 loc. loc. cit. Thorpe, cit. ; see also L'Hote, 7 loc. cit. Boscoe, loc. cit. ; Prandtl and Bleyer, 8 Thorpe, loc. cit. Brown and Snyder, J. Amer. Chem. Soc., 1925, 47, 2671. 10 Koppel and Kaufmann, Zeitsch. anorg. Chem., 1905, 45, 355. 46 VANADIUM, NIOBIUM, AND TANTALUM. that the chlorine atoms are directly attached to the vanadium atom.

The salt formulated thus : . copper is, therefore, Cu<^ j>VOCl.CuV0 3

The reaction with magnesium oxide proceeds differently and gives magnesium hexavanadate, MgaVgO^.lOHgO. 1 Vanadium Oxydichloride, V0 2C1 2.8H 20. By the action of dilute aqueous hydrochloric acid on the two isomeric hydrates of hypovanadic acid, two isomeric forms of this compound have been prepared. The green hydrate gives rise to deep green crystals, and the rose hydrate 2 to blue crystals of the same empirical composition. Their constitutions are a matter of doubt. 3 In addition to the above, two other vanadium oxychlorides have 4 been reported : V 2 2C1 4.5H 2 or 2V0 2 .4HC1.3H 2O, and V 2 3C1 2.4H 2 5 or 2V0 2.2HC1.3H 20. Thermochemical Considerations. The heats of formation of the three anhydrous chlorides of vanadium have been determined by 6 combustion in a bomb calorimeter and are found to be as follows :

(i) V (solid)+ Cl a (gas)=VC! 2 (solid) +147,000 (4000) calories, (ii) V (solid) +4Cl a (gas)=VCl 3 (solid) +187,000 (8000) (iii) V (solid)+2C! 2 (gas)=VCl 4 (liquid) +165,000 (4000)

It is obvious that the combustion of a molecule of vanadium with an increasing number of molecules of chlorine is not accompanied by a gradually increasing evolution of heat. The figures show that the formation of vanadium tetrachloride (liquid) from vanadium trichloride (solid) and chlorine (gas) proceeds endothermically :

(iv) VC1 3 (solid)+|Cl 8 (gas)=VC! 4 (liquid) -22,000 calories.

Vanadium tetrachloride is, in fact, stable only at high temperatures. The last figure is, however, unreliable, since it is considerably affected by (a) the experimental errors involved in the reactions (ii) and (iii) above, and (b) the heat of liquefaction of vanadium tetrachloride, which is at present unknown. The heat of formation of vanadium oxytrichloride is given by the equation :

(v) V (solid)+|0 2 (gas)+fd 2 (gas)=VOCl 3 (liquid) +200,000 (4000) calories. VANADIUM AND BROMINE.

is Vanadous Bromide, vanadium tribromide, VBr 3 , conveniently obtained synthetically. When powdered vanadium is gently warmed with pure, dry bromine, combination takes place readily, considerable heat is evolved and the tribromide is formed. Ferrovanadium alloy 7 gives the same product. It is also obtained by the action of pure,

1 Cuttica, Tarchi, and Alinari, Gazzetta, 1923, 53, [i], 189. 2 Gain, Ann. Chim. Phys., 1908, [viii], 14, 224. 3 Compare the monohydrates of Hypovanadic Oxide, p. 50. * J. Crow, Ghem. Soc., 1876, 30, 453 ; Perrakis, Compt. rend., 1927, 184, 1430. 6 Ditto, ibid., 1886, 102, 1310. 6 Ruff and Friedrich, Zeitsch. anorg. Chem., 1914, 89, 279. 7 Meyer and Backa, ibid., 1924, 135, 187. COMPOmSTDS OF VANADIUM. 47

dry bromine vapour on either vanadium nitride, VN, or a mixture of and 1 or on vanadium vanadous oxide, V 2 3 , charcoal, pure carbide, 2 . is a dark or V 4C 3 It green greyish-black, amorphous, deliquescent substance, which decomposes spontaneously with evolution of bromine at ordinary temperatures, giving a dark liquid. It dissolves in water without evolution of bromine to give a green solution, which possesses the same general properties as a solution of vanadium trichloride. On being into a mixture of gently heated in air it is rapidly converted vanadous vanadium V . oxide, V 2 3 , and pentoxide, 2 5 is the The hexahydrate, VBr 3 .6H 20, prepared by dissolving anhydrous tribromide in air-free water and concentrating, first on a water-bath 3 and subsequently in vacuo. It can also be prepared by dissolving in concentrated acid out vanadous hydroxide, V(OH) 3, hydrobromic 4 of contact with *air, or by the electrolytic reduction of a solution of O in acid. 5 vanadium vanadium pentoxide, V 2 5 , hydrobromic Hydrated tribromide is a green, hygroscopic, crystalline powder, which is less less stable the chloride. easily crystallisable and than corresponding The colour of its solution in water varies from brown to yellow according of acid rise to a coloration to the concentration ; addition gives green which changes to blue as oxidation to the tetravalent state ensues. is Hexammino-vanadium Tribromide, [V(NH 3 ) 6 ]Br 3 , prepared by the action of liquid ammonia on vanadium tribromide. Its properties and reactions are similar to those of the corresponding chlorine compound (see p. 42). VBr has not Hypovanadic Bromide, vanadium tetrabromide, 4 , hitherto been isolated, but it is of interest to note that a double salt of .7H has been obtained anti- composition VBr 4.SbBr 3 2O by dissolving mony tribromide and vanadium pentoxide in hydrobromic acid and 6 adding bromine.

Vanadium Oxybromides. Vanadium Oxymonobromide, VOBr, is prepared by the decom- VOBr in vacua at 360 C. It forms position of vanadyl dibromide, 2, C. violet, octahedral crystals, density 4-000 at 18 On being heated vanadous in vacuo at 480 C. it yields vanadium tribromide and oxide, solvents. 7 . insoluble in water and the usual V 2O 3 It is almost organic VOBr is obtained bromine Vanadyl Dibromide, 2 , by passing of S Br and vapour or, preferably, a mixture sulphur bromide, 2 2, bromine over a mixture of vanadium pentoxide and sulphur at a red heat the is heated in vacuo at 240 C., the vanadyl ; product whereupon 8 dibromide is obtained as a yellow powder. An alternative method

vanadium VOBr , of preparation consists in heating oxytribromide, 3 to 180 C. 9 Vanadyl dibromide is no doubt present in the blue solution V0 is dissolved in which results when hypovanadic oxide, 2? hydrobromic acid.

1 Boscoe, J. Chem. Soc., 1871, 24, 26. 2 Buff and Lickfett, Ber., 1911, 44, 2534. 3 Meyer and Backa, loc. cit. 4 Locke and Edwards, Amer* Chem. J., 1898, 20, 594. 6 Piccini and Brizzi, Zeitsch. anorg. Chem., 1899, 19, 394. 6 Weinland and Feige, Ber., 1903, 36, 260, 7 Buff and Lickfett, loc. cit. 8 Buff and Lickfett, loc. eft, 9 Boscoe, loc. cit. 48 VANADIUM, NIOBIUM, AND TANTALUM.

Vanadyl dibrornide is a hygroscopic, unstable compound. It dissolves in water to give a blue solution. On being heated in air it forms vanadium C. it pentoxide ; in vacua at 240 partly sublimes and partly decomposes, with evolution of bromine and formation of a violet residue of vanadium oxymonobromide, VOBr. The dielectric constant 1 has been investigated. No double salts of vanadyl dibromide have been prepared. is Vanadium Oxytribromide, VOBr 3 , produced when pure, dry bromine is over vanadous heated to redness. passed oxide, V 2 3 , Yellowish-white vapours are evolved which condense to a deep red, hygroscopic liquid, density 2-9673 at C. It decomposes slowly at ordinary temperatures into vanadyl dibromide and bromine, but distils without decomposition at 130 to 130 C. under 100 mm. pressure. It 2 is . much less stable than vanadyl dibromide, VOBr 2 is An oxybromide of possible constitution V0 2Br 3.5H 2 known, but the constitution of this compound is a matter of doubt. It is obtained in dilute by dissolving hypovanadic oxide, V0 2 , aqueous hydrobromic acid. Hydrochloric acid under similar conditions yields the compound V0 2C1 2 .8H 20.*

VANADIUM AND IODINE.

4 Iodine does not react with finely divided vanadium, nor with the 5 or . nitride, VN, vanadous oxide, V 2 3 Anhydrous vanadium tri- iodide has not as yet been prepared. Hydrated Vanadium Tri-iodide, VI 3.6H 20, is prepared by a solution of in reducing, electrolytically, vanadium pentoxide, V 2 5 , until acid hydriodic acid, the product becomes green ; more hydriodic is then added and the whole allowed to stand over lime and concentrated sulphuric acid at C. Small green needles separate, which have the same crystalline form as the hydrated trivalent halides of titanium, iron, and chromium. These crystals are extremely hygroscopic and deliquesce 6 in air to a brown liquid which is extremely unstable. Vanadium Oxyiodides. No definite oxyiodides of vanadium are known, although several substances of varying composition have been 7 obtained by the action of vanadium pentoxide on hydriodic acid. Vanadoiodates and vanadoperiodates are described on p. 90.

VANADIUM AND OXYGEN.

The several oxides of vanadium have already been referred to in the section describing the general properties of vanadium compounds (see p. 30). They are set out in the table on p. 33. The thermal changes involved in their formation are discussed collectively on p. 32. Vanadium Suboxide, V 20. The existence of this oxide is extremely doubtful. It is said to be the brown substance first formed

1 Loomis and Schlundt, J. Physical Chem., 1915, 19, 734. 2 Kosooe, J. Ohem. 8oc., 1871, 24, 23. 3 Gain, Ann, Ckim, Phys., 1908, [vizi], 14, 224. 4 Meyer and Backa, Zeitsch. anorg. Chem., 1924, 135, 187. 5 Compare preparation of Vanadium Tribromide, p. 46. 6 Piccini and Brizzi, Zeitsch. anorg. Chem., 1899, 19, 399. 7 see also Ann. Chim. Ditte, Compt. rend., 1886, *Q2 ? 1310 ; Gain, Phys,, 1908, [viii], 14, 224. COMPOUNDS OF VANADIUM. 49

1 2 when vanadium is gently heated in the air. Hittorf states that it is formed when vanadium * s pentoxide, V 2O5 , reduced with aluminium by the Goldschmidt process, but other investigators have found that the product so obtained is a mixture of vanadium and hypovanadous oxide, VO. 3 or is Hypovanadous Oxide, VO, vanadium dioxide, V 2O 2 , the lowest definite oxide of vanadium. Vanadium surpasses uranium in its power for combining with oxygen, and, as has been previously pointed out (see p. 16), it is an extremely difficult matter to remove the last atom. Hence this oxide, in addition to a oxygen " " enjoying separate existence, is present also as the vanadyl radical in a large number of vanadium compounds. Hypovanadous oxide has been prepared in crystalline form by reduction of the pentoxide in a magnesia crucible 4 with hydrogen at 2500 C. under a pressure equal to 75 atmospheres. It is more conveniently obtained by heating vanadium oxymonochloride, VOC1, at a red heat in a stream of hydrogen until all the chlorine is the of this reaction is a 5 removed ; product black, amorphous powder. A metallic, grey form is obtained, admixed with charcoal, when the of are mixed vapours vanadium oxytrichloride, VOC1 3 , and hydrogen 6 passed over strongly heated charcoal. Density at 15 C., 5-758. Hypovanadous oxide resembles the metal in many of its properties. It is insoluble in water, but dissolves in acids without evolution of hydrogen to yield the lavender-coloured solutions which are characteristic of solutions of hypovanadous salts. These salts are, however, most conveniently prepared in solution by electrolytic reduction in an inert atmosphere of solutions of vanadium pentoxide in the various acids. 7 Hypovanadous salts are isomorphous with salts of divalent iron, chromium, and manganese. On being treated with caustic alkalis, a brown of is precipitate hypovanadous hydroxide, V(OH) 2 , obtained, which rapidly oxidises to the greyish-green vanadous hydroxide, V(OH) 3 . is as a Vanadous Oxide, vanadium sesquiojcide, V 2O 3 , obtained black or or greyish-black powder when vanadium pentoxide, V 2O5 , is heated in a of ammonium metavanadate, NH 4V0 3 , stream hydrogen 8 at 900 to 1000 C. Ignition of the pentoxide in a current of pure carbon monoxide also results in the formation of vanadous oxide.9 Its density at 18 C. is 4-870, and m.pt. 1965 C. The electrical resis- 10 tivity of the solid has been studied by Friederich, and the magnetic behaviour by Perrakis. 11 Vanadous oxide is stable in the absence of air to a white heat it melts up temperatures approaching ; unchanged in hydrogen. In warm air it glows and oxidises rapidly to the pent- to air at for several oxide ; on being exposed ordinary temperatures 12 months it yields indigo-blue crystals of hypovanadic oxide, VO 2 .

1 Roscoe, J. Ghem. Soc., 1868, 21, 322. 2 Hittorf, Physical Zeitsch., 1902, 4, 196. Koppel and Kaufmann, Zeitsch. anorg. Chem., 1905, 45, 352. Newbery and Pring, Proc. Roy. Soc., A, 1916, 92, 282. Wedekind and Horst, Ber., 1912, 45, 262. Roscoe, Phil Trans., 1868, 158, 1. Zeitsch. Piecini, anorg. Ghem., 1899, 19, 204 ; Piccini and Marino, ibid., 1902, 32, 55. Compare Newbery and Pring, loc. cit. Mdivani, Ann. Chim. anal., 1907, 12, 305. 10 Friederioh, Zeitsch. Physik, 1925, 31, 813. 11 Perrakis, Oompt. rend., 1925, 185, 111. 12 loc. Roscoe, cit. ; Friederich and Sittig, Zeitsch. anorg. Chem., 1925, 145, 131.

VOL. vi. : m. 4 50 VANADIUM, NIOBIUM, AND TANTALUM.

Trituration of vanadous oxide with water results in a black, colloidal solution which is very stable in the absence of air. 1 The oxide reacts with ammonia at a red heat to produce vanadium nitride, VN, and is attacked by chlorine at lower temperatures to give vanadium oxytrichloride, VOC1 3 . The latter reaction was utilised by Briscoe and Little for the preparation of the vanadium oxytrichloride which they used for the determination of the atomic weight of vanadium (see p. 24). at Vanadous oxide combines with uranium trioxide, U0 3 , 600 C., 2 forming a green, normal vanadium uranate. The oxide is attacked by nitric acid and hydrofluoric acid, but is insoluble in other acids and alkalis. Acid solutions of the oxide are conveniently prepared either (a) by reducing acid solutions of vanadium pentoxide with metallic magnesium, or (&) by oxidising in the air solutions of hypovanadous salts. Addition of ammonium hydroxide or of hydroxides or carbonates of the alkalis to the acid solutions throws down a greyish- fiocculent of vanadous green precipitate hydroxide, V(OH) 3 , which rapidly becomes brown as oxidation proceeds. When washed in an inert atmosphere vanadous hydroxide forms a starting material for the preparation of salts of trivalent vanadium.

or . Hypovanadic Oxide, V0 2 , vanadium 'tetroxide, V 2 4 When or is treated in vanadyl sulphate, VOS0 4 , vanadyl dichloride, VOC1 2 , solution with an alkali hydroxide or carbonate, not in excess, a greyish- white precipitate is obtained which, after being washed in an inert atmosphere, is found to consist of an amorphous, hydrated hypovanadic oxide, V0 2.o?H 20. The number of molecules of water varies with the method of drying. When it is dried over concentrated sulphuric acid at room temperature its composition is expressed by the formula .7H heated to 100 .3H 0. 2V0 2 2 ; when C., by 2V0 2 2 A compound of .2H which would to has composition VO 2 2O, correspond V(OH) 4 , not been isolated. Reduction of vanadium pentoxide with sulphur dioxide and concentration in an atmosphere of carbon dioxide yields 3 a red, crystalline monohydrate, V0 2 .H 2O, which becomes green on exposure to air. The green product was originally thought to be an 4 isomeride of the red compound, but more recently the colour change 5 has been shown to be due to partial oxidation. The monohydrate loses its water when heated in an inert atmosphere at temperatures between 200 and 250 C. It dissolves in sulphuric acid to yield an azure-blue solution, with evolution of 12,620 calories of heat per gm.-mol. Hypovanadic oxide is most conveniently obtained by heating any of the foregoing hydrates in the absence of air. It is also formed (a) from vanadium pentoxide by heating in admixture with the trioxide, carbon, 6 or oxalic acid, and (b) from vanadous oxide by prolonged exposure to air.7 The oxide has been variously described, according to the method of preparation, as consisting of a black or green, amorphous powder, steel- blue Its is 1637 C. crystals or indigo-blue crystals. m.pt. ; density

1 Wegelin, Kolkid. ZeitscL, 1914, 14, 68. 2 and Zeitsck. 88. Tammann Rosenthal, anorg. Chem., 1926, 156, 20 ; compare p. 8 Gain, Compt. rend., 1906, 143, 823, 1154. 4 Gain, ibid., 1907, 146, 403. 5 Parisi, Gazzetta, 1926, 56, 843. 6 1 Berzelius, Pogg. Annalen, 1831, 22, ; Guyard, Bull Soc. chim. t 1876, [ii], 25, 351 ; J. Ditte, Cornet, rend., 1885, 101, 1487 ; Crow, Chem. Soc., 1876, 30, 454. 7 Boscoe, ibid., 1868, 21, 338. COMPOUNDS OF VANADIUM. 51

1 2 at 13 C., 4-399. Its magnetic properties have been investigated. It is oxidised to the pentoxide on being heated in air and by nitric acid. Hypovanadic oxide is amphoteric. It dissolves slowly in acids other than nitric acid to give solutions of vanadyl salts, as, for example,

. solutions are more vanadyl sulphate, VOSO 4 These conveniently prepared by the partial reduction of acid solutions of vanadium pentoxide. The reducing agent employed must, of course, be one with which reduction ceases on the formation of the tetravalent salt; the sulphur dioxide, nitrous acid, phosphorous acid, halogen acids, and, under definite conditions, sugar, and alcohol, are available for the are become after purpose. The solutions so prepared blue, green salts on concen- exposure to the air, and deposit blue hydrated being salts are either brown or trated ; the anhydrous vanadyl green. Vanadyl compounds undergo reoxidation to vanadic acid by the action of bromine, potassium permanganate, potassium ferricyanide or sodium oxide to be too peroxide in alkaline solution. Hypovanadic appears rise to stable salts of the in which four weakly basic to give type VX4 , acid groups are attached to a vanadium atom. The tetrafluoride and tetrachloride of vanadium, VF 4 and VC1 4 , have been prepared, but they to and VOC1 The undergo very ready hydrolysis VOF 2 2 respectively. formula and ionise in solution vanadyl salts have the general VOX 2 , to give the vanadyl cation, [VO]", which is analogous, for instance, 3 acid unites with with the uranyl cation, [UO 2]*'. Hypovanadic many organic acids formic, acetic, oxalic, malonic, tartaric, citric, benzoic, and to cases, crystalline succinic, salicylic * 0Jpt 4 foTm^p^^L " compounds.

v soluble in vanadic oxide dissolves In addition to being ^fts a hy^o in excess of hot alkalis. Cooling these" solutions in the absence of air or vanadites. Insoluble gives rise to a series of crystalline hypovanadates vanadites are formed by double decomposition of a metallic salt and the formula R V . an alkali vanadite ; they usually possess general 2 4 9 salts of the acid H V O . #H 2O, and are perhaps hypothetical 2 4 9 They are comparable to the salts produced by other weakly acid oxides, e.g. has uranates, metatungstates and borates, in that the acid oxide undergone condensation to a poly-acid. Vanadites are either black or brown in the solid state. They become oxidation to the green in damp air, and in solution readily undergo of tannin solution a corresponding vanadates. Addition produces blue-black coloration. Lead acetate throws down a curdy, brown silver nitrate a black, Accord- precipitate ; gives crystalline precipitate. 5 of a silver in which ing to Crow the latter has the composition vanadite, acid oxide in combination there are, however, only two molecules of but to with one molecule of the base, Ag 2O.2V0 2 ; according Koppel 6 of silver and Goldmann the precipitate consists of a mixture vanadate,

1 loc. cit. Roscoe, Zoc. cit. ; Friederich and Sittig, 2 Perrakis, Compt. rend., 1925, 185, 111. 3 See this series, Vol. VII., Part III. (1926), p. 287. * 1831, 22, 1. Gain, Ann. Chim. Phys., 1908, [viii], 14, 224 ; Berzelius, Pogg. Annakn, s Crow, J. Ohem. Soc., 1876, 30, 462. 6 Koppel and Goldmann, Zeitsch. anorg. Chem., 1903, 30, 300. 52 VANADIUM, NIOBIUM, AND TANTALUM. silver vanaditc, and metallic silver. The following vanadites have been prepared :

Ammonium Vanadite, (NH 4 ) ?V 4 9 .3H 20. A solution of a vanadyl is in salt, e.g. vanadyl dichloride, VOC1 2 , slowly added calculated quantity to boiling concentrated ammonia solution. On cooling in the absence of air, golden-brown, glistening needles or scales are deposited, readily soluble in water but insoluble in alcohol, ether, or ammonia. 1 The substance gradually loses ammonia on exposure to air. Barium Vanadite forms two hydrates, BaV4O 9 .4H 2O and BaV 4O 9 . 5H 2O, which are obtained by the action of excess of baryta water on a solution of vanadyl dichloride. 2 They are brown, amorphous substances, easily soluble in nitric and hydrochloric acids. or is a Lead Vanadite, PbV 2O 5 PbO.2V0 2 , obtained as brown, curdy 3 precipitate by the action of lead acetate on potassium vanadite. Potassium Vanadite, K 2V4 9 .4H 2 0. A boiling solution of a vanadyl salt is poured into an excess of a 10 per cent, solution of caustic potash and the product cooled in the absence of air. Alternatively, a solution of ammonium vanadite is warmed with caustic potash. Potassium vanadite forms a brown, pearly, glistening mass, and is similar to the 4 salt . ammonium in its general properties. A heptahydrate, K 2V 4 9 5 7H 2O, has been reported. Sodium Vanadite, Na 2V 4O 9 .4H 2O, forms golden-brown needles, and 6 is prepared similarly to the corresponding potassium salt. Brown, a crystalline scales of heptahydrate, Na 2V 4O9 ,7H 20, have also been 7 reported.

Intermediate Oxides. Oxides which are intermediate between and vanadium are known. hypovanadic oxide, V0 2 , pentoxide, V 2 5 , By the partial reduction of vanadium pentoxide, or by the partial oxidation of one of the lower oxides, there have been prepared a number of oxides which are best considered as being formed by the combination of the acidic vanadium pentoxide with a lower basic oxide in varying molecular proportions. These oxides react with alkalis, and yield a series of salts called vanadyl vanadates, intermediate in composition between the vanadites and the vanadates.

2VO.V 2 5 is produced as a dark blue, crystalline powder when either vanadium pentoxide or ammonium metavanadate is heated with excess of powdered arsenic, or when ammonium metavanadate is reduced with sulphur dioxide at a red heat. It dissolves in nitric acid to a blue solution. 8 is 6VO 2 .V 2O5 obtained as a dark blue or black powder with a metallic 9 lustre by dissolving vanadium pentoxide in caustic potash. 2V0 2.V 2O5 also results 'as deep blue crystals on heating ammonium metavanadate which has previously been fused and cooled. It is stated that the fused substance does not furnish vanadium pentoxide on being decomposed.10 The residue is extracted with concentrated ammonium hydroxide solution and the oxide precipitated by addition of water. 1 Zeitsch. JCoppel and Goldmann, anorg. Ghem., 1903, 36, 261 ; Crow, J. Chem. Soc., 1876, 30, 462; Mawrow, Zeitsch. anorg. Chem., 1907, 55, 147. 3 loc. cit. loc. cit. loc. Koppel and Goldmann, ; Crow, Crow, cit. 5 Koppel and Goldmann, loo. cit. Crow, loc. cit. 7 Koppel and Goldmann, loc. cit. Crow, loc. cit. Ditte, Compt. rend., 1885, 101, 698, 1487. 10 Manasse, Chem. Zentr., 1886, 773. Compare p. 54. COMPOUNDS OF VANADUJM. 53

The product is slightly soluble in concentrated nitric acid and is readily attacked by hydrochloric acid. It is a little doubtful if the oxide prepared in this manner is not an ammonium vanadyl vanadate. 1 dark .3V .SII A green hydrate, GVO 2 2O5 2O, has been obtained by the

of . gentle ignition ammonium vanadyl vanadate, 3(NH 4 ) O.4YO 2.4V 2O 5 2 6H 2O. VO .V .4H O results on the of 2 2O 5 2 exposure vanadous oxide, V 2O 3, to air for several months. 3 Several other purple, green, or orange intermediate oxides of doubtful 4 composition have been reported.

Vanado-vanadates.

These salts are made by reducing hot solutions of alkali vanadates with dioxide after addition of acetic sulphur ; acid they can be salted 5 out by addition of alkali acetate. The following are known :

(NH 4 ) 2O.4VO 2.2V 2O 5 .14H 2O . Greenish-black crystals.

.4V .6H . lustre. 3(NH 4 ) 2O.4VO 2 2O5 2O Purple crystals with metallic

(NH 4 ) 2O.2VO 2.4V 2O 5 .8H 2O . Green crystals. 2K 2O.4V0 2 .V 2O5 .6H 2O . . Greenish-black crystals. 5K 2O.4VO 2 .4V 2O5 .H 2O . . Purple crystals with metallic lustre. 2Na 2O.4VO 2.V 2O 5 .13H 2O . Black, shining, hexagonal prisms. In addition to the above, several vanado-vanadates have been prepared by fusing alkali carbonates, borates, phosphates, silicates, arseiiates, with vanadium in air. The fused mass etc., pentoxide" " evolves oxygen during cooling, and spitting of the material takes place. The phenomenon recalls the behaviour of solidifying silver. On again heating in air, oxygen is re-absorbed and acid vanadates are produced. The evolution of oxygen is accompanied by reduction of a portion of the vanadium pentoxide content with formation of the vanado-vanadate. The reaction is represented :

^R' 20.z/V 2 5 ^:a?Il- 20.(2/-s)V 2 5 ^V 2 Hot. Cold.

The oxygen is liberated between the upper limit of stability of the vanado-vanadate and the point of solidification of the fused mass. The proportion of oxygen evolved is independent of the anion with which the metal is combined. The alkali salts can be substituted by thallium compounds but not by alkaline earth oxides. Investigation shows that for each base there is a definite vanadium pentoxide content for which the evolution of oxygen is a maximum. 6 Phase diagrams for the systems V 2O5 Na 2O, V 2O5 -K 2O, V 2O 5 -Li 2O, and V 2O 5 T1 2O show that in each case three definite compounds are formed, corresponding to the meta-, pyro-, and orZ&o-vanadates. By the fore- 7 going method the following salts have been prepared: 6Li 2O.2VO 2. .4V 11V2O5 ; 4Li 2O.2VO 2.7V 2O 5 ; Na 2O.2VO 2.5V 2 5 ; Na 2O.2V0 2 2O 5 ;

1 2 Ditte, loc. cit. Brierley, J. Chem. Soc., 1886, 49, 30. 3 * Brierley, loc. cit, Berzelius, Pogg. Annalen, 1831, 22, 1. 5 Brierley, loc. cit. ; Gibbs, Amer. Chem. J. 9 1886, 7, 209. 6 Canneri, Gazzetta, 1928, 58, 6. 7 Zeitech. Canneri, loc. cit. ; Prandtl, Ber., 1905, 38, 657 ; Prandtl and Murschhauser, 744. anorg. Chem., 1907, 56, 173 ; 1908, 60, 441 ; Hautefeuffle, Compt. rend., 1880, 90, 54 VANADIUM, NIOBIUM, AND TANTALUM. .8V K O. Na 2O.V0 2.2V 2 5 ; 3Na 2O.2V0 2.5V 2 5 ; K 20.2VO 2 2 5 ; 2

.4V O O.V0 .2V O ; ; K 2V02 .5V 2 5 ; 2K 20.2V0 2 .9V 2 5 ; K 20.2V0 2 2 5 2 2 2 5 Ag 20.2V0 2.5V 2 5 . Vanado-vanadates can also be obtained by fusing alkali vanadites with metavanadates, when no oxidation or loss of oxygen takes place. 1 .V O and K O. This process has furnished two salts, Na 20.2V0 2 2 6 2

2V0 2 .V 2 5 .

Vanadium Pentoxide, vanadic oxide, or vanadic anhydride, V 2 5 . This is one of the commonest of the compounds of vanadium, and other constitutes the starting material for the preparation of many vanadium compounds. Its manufacture on the large scale, as a stage in the industrial production of ferrovanadium and metallic vanadium, method for its has been described on pp. 14 el seq. A laboratory extraction from vanadium minerals is as follows : The mineral is roasted strongly with a mixture of sodium carbonate and potassium nitrate. The aqueous extract of the fused product, which contains alkali vanadates together with the alkali salts of other acids, is first neutralised with nitric acid, to precipitate silicic acid and aluminium hydroxide, and then concentrated until most of the potassium nitrate chloride is crystallised out. The mother-liquor is heated with barium and ammonium hydroxide, whereupon barium vanadate, chromate, out. phosphate, arsenate, molybdate, tungstate and sulphate separate Treatment of these barium salts with sulphuric acid liberates the free acids, which are carefully neutralised with ammonium hydroxide and concentrated. The further addition of ammonium hydroxide at this stage yields white, crystalline ammonium metavanadate, NH 4y0 3 , heated in a which is purified by repeated crystallisation. On being decom- platinum crucible with access of air, ammonium metavanadate poses with formation of red, amorphous vanadium pentoxide, ammonia,

and water : 0. 2NH 4V0 3 =V 2 5 +2NH 3 +H 2

By dissolving the residue in caustic soda and repeating the precipitation and decomposition of ammonium metavanadate, a pure product can be obtained. It is necessary for the ammonium metavanadate to be quite free even from traces of organic matter, chlorides, phosphates, oxides is etc., as otherwise a mixture of the pentoxide and the lower obtained on ignition. Chemically pure vanadium pentoxide is alternatively prepared by pre- insoluble from a neutral solu- cipitating mercurous vanadate, HgV0 3 , of tion of a vanadate, and distilling off the mercury, or by ignition vanadium salts of volatile acids, for example, vanadium oxytrichloride,

. oxidation of of the lower VOC1 3 The oxide also results from the any or oxides, or by the electrolysis of a solution of sodium vanadate copper a cell the last method a of vanadate, using divided ; yields product 2 98 per cent, purity. By addition of mineral acids to solutions of vanadates, or by the hydrolysis of vanadium oxytrichloride, a reddish-brown, gelatinous precipitate of hydrated vanadium pentoxide is obtained. This is very

1 Canneri, Gazzetta, 1928, 58, 6. 2 MectrocJiem. 8oc. Bleecker, Met Chem. Eng., 1911, 9, 601 ; Fischer, Trans. Amer. t 1916, 30, 175. COMPOUNDS OF VANADIUM. 55

similar in to ferric appearance hydroxide, Fe(OH) 3 , and on examination is found to consist of very fine particles which cannot be washed free from the mother-liquor without undergoing peptisation to a colloidal solution. The preparation and properties of colloidal vanadium pentoxide are dealt with on p. 58. Vanadium pentoxide can be obtained in two modifications : (a) red crystalline; (b) red or yellow amorphous. (a) The red, crystalline variety is obtained from the amorphous form by fusing in a porcelain or platinum dish. After cooling, the mass is found to have decreased in volume and to have solidified to an intensely coloured, glistening mass of ruby-red, rhombic crystals, from 1 3 to 4 cm. long and from 2 to 3 mm. broad. Bleecker describes the solid mass as consisting of needles, arranged parallel and extending to each surface the of inward perpendicularly ; ends the crystals meet at 45, and a vertical section has the appearance of three pyramids 2 each with its base extending outwards. The X-ray diffraction pattern has been examined.3 An alternative method for the preparation of crystalline vanadium pentoxide consists in heating a mixture of the amorphous oxide and calcium fluoride to red heat in an open crucible over which is suspended another crucible, inverted, to act as a receiver. The inside of the latter becomes coated with shining, needle-shaped, yellow crystals, which also become reddish-brown on being heated.4 Evaporation of the hydrochloric acid solution of the red, amorphous 5 oxide may also give rise to crystals. of oxide at 20 C. is 3-56 6 The density the crystalline ; other deter- 7 minations gave 3-32 at 15 and 3-357 at 18 C. The oxide is not hygroscopic even after prolonged exposure under ordinary conditions. Its 8 saturated solution in water contains 50 mgm. per litre, but the solubility is affected by the state of aggregation of the solid and by its tendency trituration has a sol which contained 910 to form a hydrosol ; given 9 mgm. per litre. (b) The red, amorphous form of vanadium pentoxide is the form most frequently met with in the laboratory. Its preparation has been 10 11 described above. It melts at 658 or 675 C. to a dark red liquid, it but is not volatile even at high temperatures ; can be vaporised only 12 in the electric furnace. The fused solid conducts electricity, with 13 the electrical has formation of hypovanadic oxide, VO 2 ; conductivity 14 been measured. The oxide absorbs water on exposure to the air, the 1 1487 K. Akad. Wiss. Ditte, Oompt. rend., 1885, 101, 698, ; Safarik, Sitzungsber. Wien, 160 Prandtl and Zeitsch. 1858, 33, 1; Nordenskiold, Pogg. Annalen, 1861, 112, ; Bleyer, anorg. Chem., 1910, 67, 257. 2 also J. Chem. Bleecker, Met. Chem. Eng., 1910, 8, 666 ; see Thorpe, Soc., 1880, 37, 348. Rinne, Zeitsch. Kryst. Min., 1924, 60, 55. 5 4 Prandtl and Manz, Ber., 1911, 44, 2582. Bleecker, loc. cit. 6 Safarik, Sitzungsber. K. Akad. Wiss. Wien, 1863, 47, 256. 7 Prandtl and loc. cit. Wedekind and Horst, Ber., 1912, 45, 265 ; Bleyer, 8 Ditte, loc. cit. Kolloid. 1914, 67. w Wegelin, Zeitsch., 14, Carnelly, J. Chem. Soc., 1878, 33, 279. 11 Canneri, Gazzetta, 1928, 58, 6. 12 J. Moissan and Holt, Oompt. rend., 1902, 135, 78, 493 ; compare Read, Chem. Soc., 1894, 65, 313. 13 Buff, Annalen, 1859, no, 276; Bleekrode, Wied. Annalen, 1878, pi], 3, 171. 14 Friederich, Zeitsch. Physik, 1925, 31, 813. 56 VANADIUM, NIOBIUM, AND TANTALUM. amount taken up depending on the vapour-pressure of the surrounding 1 atmosphere. The water of hydration can be removed by careful 2 heating to 300 C. The dried oxide feels greasy to the touch and discolours the skin slightly. Its saturated solution contains between 3 0-90 and 1*25 gram per litre. The density of a solution containing 0-90 gram per litre is 0-9988 at 20 C. and 0-9978 at 26 C. The adsorptive power of vanadium pentoxide for helium, oxygen, hydrogen, carbon monoxide, and carbon dioxide at different temperatures has been measured.4 its is a The yellow, amorphous variety is unstable and identity little doubtful. It is stated to be obtained sometimes from the red, amorphous form by evaporation of a solution of the latter in hydrochloric acid, and it may also result from the ignition of ammonium metavanadate or from the decomposition of vanadates by acids. According to Bleecker it is most conveniently prepared by the electrolytic decom- 5 It brick red on position of copper vanadate. becomes being heated, and is similar to the red variety in its general properties, except that and less soluble its saturated it appears to be less hygroscopic ; aqueous 6 solution contains between 300 and 400 mgm. per litre. Vanadium pentoxide dissolves in acids, both organic and inorganic, 7 to form vanadyl or unstable vanadic salts, and in alkalis to produce and The ortho-, pyro-, meta- 5 poly-vanadates. physico-chemical changes involved when vanadium pentoxide is heated with various 8 basic oxides in the powder state have been investigated by Tammann. On being digested with liquid ammonia slow absorption of ammonia takes the of the has not been place ; composition product definitely 10 established. 9 The oxide also dissolves in alcohols to produce esters, and combines with methylamine and ethylamine to form compounds O where R the radical. 11 of the type 2(R.NH 2 ).V 2 5 , represents alkyl Vanadium pentoxide is a powerful oxidising agent, and undergoes reduction in stages depending on the reducing agent employed and on other conditions of the process. In the absence of moisture it is reduced red and to hypovanadic oxide, V0 2, by sulphur dioxide, phosphorus, ammonia, while dry hydrogen, carbon monoxide, sulphur, and potassium cyanide, at varying temperatures and atmospheric pressure, yield

. at C. 75 vanadous oxide, V 2O 3 Hydrogen 2500 and atmospheres acid solution reduction pressure yields hypovanadous oxide, VO. In of vanadium pentoxide to the tetravalent state, which is characterised by the appearance of a blue colour, can be effected with quite a large number of reducing agents : sulphur dioxide, hydrogen chloride, 12 hydrogen bromide, hydrogen iodide, hydrogen sulphide, nitrous acid,

1 Freundlich and Leonhardt, Kolloidchem. Beihefte, 1915, 7, 188. 2 Bleecker, loc. cit. 8 Ditte 0-8 litre. Gessner, Kolhidchem. Beihefie, 1924, 19, 213 ; (loc. cit.) gives gram per 4 Benton, J. Amer. Ohem. Soc, 9 1923, 45, 893. 5 loc. cit. Ditte, loc. cit. ; Bleecker, 6 Bleecker, loc. cit. 7 Fischer, Trans. Amer, Electrochem. Soc., 1916, 30, 190. 8 see also de Atti Tammann and Halsing, Zeitsch. anorg. Chem., 1925, 149, 68 ; Carli, R. Accad. Lincei, 1925, [vi], I, 533. Ephraim and Beck, Helv. CUm. Acta, 1926, 9, 38. 10 Prandtl and Hess, Zeitsch. anorg. Chem., 1913, 82, 103 ; Hess, J. 8oc. Chem. 2nd., 1914, 33, 712. 11 Ditte, CompL rend., 1887, 104, 1844. 12 Gooch and Curtis, Amer. J. Sci., 1904, [iv], 17, 41. COMPOUNDS OF VANADIUM. 57

1 phosphorous acid, oxalic acid, tartaric acid, lactic acid, citric acid, 2 hydrazine, hydroxylamine, alcohol, formalin, sugar, ferrous sulphate, sodium thiosulphate, and mercury. Sulphur dioxide is most commonly for the reduction it works in employed ; slowly the cold but rapidly 3 when the solution is heated :

V 2 5 +S0 2 =2V0 2 +S0 3 . Excess of sulphur dioxide can be removed by boiling in an atmosphere of carbon dioxide. In the presence of suspended carbonaceous matter reduction may proceed to the trivalent stage. With many of the above-mentioned reagents reduction proceeds quantitatively, so that vanadium pentoxide can be employed for the estimation of the reduc- for and 4 ing agent, e.g. hydroxylamine hydrazine ; alternatively, these substances become available for the estimation of solutions of vanadium pentoxide and of vanadates. Dry hydrogen chloride in the presence of a dehydrating agent does not reduce vanadium pentoxide, but forms 5 vanadium oxytrichloride :

V 2 5 +6HC1^-2VOC1 3 +3H 20.

Concentrated solutions of hydrochloric acid, however, dissolve vanadium with the of an intense pentoxide production brown coloration ; addition of water gives a yellow solution which evolves chlorine on being warmed, the solution becoming blue :

V 2 5 +2HC1 2V0 2 +C1 2 +H 20. This reaction is reversible, so that for complete conversion to the tetravalent state the concentration of the hydrochloric acid must be main- of tained. Repeated evaporation the solution to dryness gives a 6 residue of hypovanadic oxide, V0 2 . Volatilisation of the vanadium 7 to a small extent as oxychlorides may also take place. Dilute hydrochloric acid has no reducing action on vanadium pentoxide. Hundes- 8 hagen has observed that the solution of vanadium pentoxide in hydrochloric acid dissolves gold and other noble metals. If the solution is neutralised, the gold is precipitated as a greyish-violet powder, which aciid. redissolves on adding more The reaction is expressed :

3VOC1 3VOC1 AuCl . ? +Au^ 2 + 3 Alkaline. Acid.

With hydrobromic acid and hydriodic acid reduction may proceed 9 to the trivalent state, and it has been shown that in the presence of acetic also vanadous acid, hydrazine produces oxide, V 2O 3 , instead of 10 hypovanadic oxide, VO 2 :

V 2 5 +H 2N.NH 2 =V 2 3 +N 2 +2H 20.

1 Browning, Zeitsch. anorg. Chem., 1894, 7, 158. 2 _Koppel and Behrendt, ibid., 1903, 35, 156. _ 3 Compare pp. 99 et seq. * and Hofmann Kiispert, Ber., 1898, 31, 64 ; Knorre and Arndt, ibid., 1900, 33, 30. 6 Ephraim, Zeitsch. anorg. Chem., 1903, 35, 66. 6 Gooch and Curtis, ibid., 1904, 38, 246 ; Aachy, J. Ind. Eng. Chem., 1909, 1, 455; Edgar, J. Amer. Chem. Soc., 1916, 38, 2369. 7 Auerbach and Lange, Zeitsch. angew. Chem., 1912, 25, 2522. 8 Hundeshagen, Chem. Zeit., 1905, 29, 799. 9 Ditz and Bardaeh, Zeitsch. anorg. Chem., 1915, 93, 97; Edgar, loc. cit. 10 Meyer and Markowitz, ibid., 1926, 157, 238. 58 VANADIUM, NIOBIUM, AND TANTALUM.

Hydrogen also reduces pentavalent and tetravalent vanadium salts 1 to the trivalent state in the presence of spongy platinum. 2 With mercury the following reaction takes place :

V 2O 5 +2Hg+3H 2S0 4 =2VOS0 4 +Hg 2S0 4 +3H 20.

The equilibria for tri-, tetra-, and penta-valent vanadium in sulphuric acid solution have heen studied by Auger. 3 Concentrated acid solutions of vanadium pentoxide are reduced to the tetravalent state by hydrogen peroxide, the peroxides of sodium, barium, magnesium, and by persulphates of potassium and ammonium. 4 Acid solutions of vanadium pentoxide give rise to pervanadic acid with hydrogen peroxide. Reduction of acid solutions of vanadium pentoxide to the tetravalent state also takes with bismuth 6 place amalgam ; magnesium gives 6 the trivalent salts of vanadium, while by using zinc, zinc coated with cadmium, electrolytically deposited cadmium, or sodium amalgam in 7 the absence of air, divalent vanadium salts are obtained in solution. Vanadous salts and hypovanadous salts are, however, much more conveniently prepared by electrolytic reduction of acid solutions of vanadium pentoxide in an atmosphere of carbon dioxide. 8 Vanadium pentoxide becomes markedly photo-sensitive when immersed in glycerol, benzaldehyde, cinnamic aldehyde, cuminol, or aqueous mannitol solution, and exposed to light. It blackens and

to . undergoes reduction, giving rise, initially, hypovanadic oxide, VO 2 With aqueous solutions of citric acid or tartaric acid carbon dioxide is 9 evolved during the change. Molten vanadium pentoxide is a corrosive substance and attacks 10 most containers even when made of platinum, fused silica, or graphite. 1 * Colloidal Vanadium Pentoxide. When a soluble vanadate is treated with mineral acids, a red, curdy form of vanadium pentoxide is precipitated, which, on being shaken with water, appears to dissolve to a red liquid. This reaction gives rise to the following usual method for

a colloidal solution : Ammonium is making metavanadate, NH4V0 3 , made into a paste with 10 per cent, hydrochloric acid of 10 per cent, concentration, and the resulting gel of vanadium pentoxide is washed repeatedly on the filter with distilled water until it assumes the colloidal form, i.e. until it is peptised, and in consequence passes through the

1 Gall and Manchot, Eer., 1925, 58, [B], 482. 2 J. Chem. McCay and Anderson, Amer. 8oc. f 1922, 44, 1018. 3 Auger, Compt. rend., 1921, 173, 306. 4 Cain and J. Auger, ibid., 1921, 172, 1355 ; Hostetter, Amer. Chem. JSoc., 1912, 34, 274. 5 Someya, Zeitsch. anorg. Chem., 1926, 152, 368. 6 Roscoe, J. Chem. Soc., 1868, 21, 322 ; Gooch and Gilbert, Zeitsch. anorg. Chem. 9 but 1903, 35, 420 ; Glassmann, J3er., 1905, 38, 600, 605 ; compare Gooch and Edgar, Amer. J. Sci., 1908, [iv], 25, 233. 7 Chapman and Law, Analyst, 1907, 32, 250 ; Treadwell, Helv. Chim. Ada, 1921, 4, 563; 1922,5,739. 8 Zeitsch. Chem. Piccini and Brizzi, anorg. t 1899, 19, 394 ; Bultemann, Zeitsch. EleUro- Zeitsch. chem., 1904, 10, 141 ; Renschler, ibid., 1912, 18, 137 ; Piccini, anorg. CTiem., Piccini and 55 see also 1899, 19, 204 ; Marino, ibid., 1902, 32, ; p, 34 (this volume). 9 Eenz, Helv. Chim. Acta, 1921, 4, 961 , Gibbons, Chem. News, 1874, 30, 267. 10 Bleecker, Met. Chem. Eng., 1910, 8, 666 ; Mdivani, Ann. Chim. anal, 1907, 12, 305. 11 A review of the history and preparation of colloidal vanadium pentoxide is given by Gessner, Kolloidchem. Beihefie, 1924, 19, 213, COMPOUNDS OF VANADIUM. 59 filter. On shaking the residue at this stage with a large quantity of 1 water, a beautiful, clear red hydrosol is obtained. Vanadium pentoxide hydrosols have been prepared also : (a) By hydrolysing the esters of vanadic acid 2 fused vanadium organic ; (b) by pouring into water 3 the of vanadium pentoxide ; (c) by passing vapours into water. The flakes of vanadium oxytrichloride, VOC139 boiling 4 pentoxide produced are treated with a large quantity of distilled water. Sols prepared by these methods vary in colour from blood red to reddish- brown, but Wegelin also prepared a canary-yellow sol by prolonged trituration of the reddish-brown crystals of vanadium pentoxide 5 obtained by slow cooling from the molten state. Vanadium pentoxide sols may contain up to 125 gram of vanadium 6 pentoxide per litre. They do not undergo change on being boiled with water, and are not precipitated by addition of alcohol. The particles are negatively charged, and on being electrolysed move towards the behaves like solutions of vanadates and vanadium anode ; the hydrosol 7 pentoxide in that it yields the ions of polyvanadic acids. The sol is very sensitive to the action of electrolytes, relatively low concentrations of which are sufficient to produce clouding within a few minutes. The 8 flocculation value as a rule is lower the greater the value of the pre- of cipitating ion. Addition of several drops of mineral acid or other electrolyte is followed by immediate adsorption of the positive ion with formation of vanadium pentoxide gel. If nitric acid has been added the gel can be reconverted into the sol with water that is, the gelation used the is reversible ; if sodium chloride or calcium chloride has been gel cannot be re-solated. When treated with suitable concentrations of electrolytes a vanadium pentoxide sol sets to a stiff jelly. In these sols there is always a small amount of the oxide in molecular solution. This portion is not thrown down by electrolytes, and passes with through a dialysing membrane. The relative flocculation values 9 some inorganic salts have been determined, and the cataphoresis at 10 small electrolyte concentrations has been measured. Stiff, transparent from its gels of vanadium pentoxide have been prepared hydrosols or u by coagulation with a suitable electrolyte, by dialysis ; hydrosols of the vanadates of manganese, iron, aluminium and zinc can also be used. Vanadium pentoxide sols can be employed to bring about coagulation colloids for ferric and alu- of positively charged ; example, hydroxide of a minium hydroxide. The amount necessary for the coagulation with given quantity of the positive colloid is very small in comparison the required quantities of arsenic trisulphide, antimony trisulphide, the colloidal and other negative colloids. It appears, therefore, that

1 Biltz, Ber., 1904, 37, 1098. 2 Riedel, Ghem. Zentr., 1914, [i], 1738. 3 Muller, Kolloid. Zeitsch., 1911, 8, 302. 4 Wegelin, Zeitsch. Chem. Ind. Kolloide, 1912, II, 25. 5 Wegelin, Kolloid. Zeitech., 1914, 14, 65. 6 loc. cit. Gessner, Zoc. cit. ; compare Wegelin, 7 Chem. 1923, I, 396. Dumanski, Kolloid. Zeitsch., 1923, 33, 147 ; Osterman, Zentr., 8 The flocculation value is the minimum concentration required for turbidity in a definite time.

fl and Freundlieh and Leonhardt, Kolkidchem. Beihefte, 1915, 7 195 ; Ghosh Dhar, J. Physical Chem., 1927, 31, 187, 649. 10 Ivanitskii and Proskurnin, Kolloid. Zeitsch., 1926, 39, 15. 11 Ghosh, Chakravarti, and Dhar, Zeitsch. anorg. Chem., 1926, 152, 399. 60 VANADIUM, NIOBIUM, A1SID TANTALUM.

electrical particles of vanadium pentoxide carry a relatively much larger 1 charge. The pentoxide sols on being treated with reducing agents furnish the sols of lower oxides of vanadium, which are also found to 2 be negatively charged. The viscosity of vanadium pentoxide sols has 3 been investigated. Vanadium pentoxide sols display peculiar optical phenomena. A sol is clear after however, the sol, on freshly prepared quite ; ageing, to be filled with being stirred and observed by reflected light, appears in it. yellow, shining streaks, as though fine crystals were floating to be When viewed by transmitted light the aged sol appears clear, further although peculiar dark streaks can be clearly seen. On being examined between crossed Nicols, the aged sol exhibits the striking the field remains dark so as the sol property of double refraction ; long or an is not disturbed, but stirring, or placing the sol in a magnetic 4 refraction electric field, causes it to become bright at once. The double produced is so strong that if a concentrated sol is caused to flow through it is able to a tube of triangular cross-section which is used as a prism, lines the red line, for instance, is resolved split up spectral ; hydrogen 5 in this manner into two oppositely polarised lines. Plates showing the and appearance of vanadium pentoxide sols under the ultramicroscope 6 in polarised light are given by Zocher. of the double refraction is A possible explanation of the cause afforded by examination of the freshly prepared and aged sols under sub- the ultramicroscope. The freshly prepared sol contains only on microns below the limits of ultramicroscopic visibility ; ageing, however, the concentration of the molecularly dispersed vanadium pentoxide decreases, and there begin to grow in the sol elongated, rod-like needles the length of which is approximately thirty times the diameter. These new particles are unquestionably crystalline, is not the and possess slow Brownian movement, but their formation of the sol is also ordinary process of crystallisation, because the ageing 7 accompanied by changes in the dielectric constant, the specific inductive 8 9 towards electro- capacity, the electrical conductivity, the sensitivity 10 rods is due to the lytes, and the viscosity. The growth of the aggre- 11 gation of non-spherical primary particles in parallel layers. With to and further ageing the red colour of the sol finally changes yellow, yellow solutions are devoid of colloidal particles. It is generally thought that the phenomenon of double refraction is due to the appearance of these ultramicroscopic needles, the longitudinal axes of which become orientated so as to be coincident with the optical axes when the sol is

1 Freundlich and Leonhardt, loc. cit. 2 Dumansld, Kolloid. Zeitsch., 1923, 23, 147. 3 and Zeitsch. Gessner, Kolloidchem. JBeihefte, 1924, 19, 213 ; Chakravarti Dhar, anorg. Chem., 1926, 152, 393. 4 Freundlich, Zeitsch. Elektrochem., 1916, 22, 27. fi Freundlich, Physikal Zeitsch., 1915, 16, 419.

see also Kolloid. Zeitsch* 9 Zocher, Zeitsch. anorg. Chem., 1925, 147, 108 ; Joachims, 220. 1927, 41, 215 ; Zocher and Jacobsohn, ibid., 1927, 41, 7 769 Purth and Kolloid. Zeitsch., Bikerman, Physical Zeitsch., 1926, 27, ; BJuh, 1924, 34, 259. 8 Errera, Bull Soc. chin. Belg., 1924, 33, 422, 432. 9 703. Dumanski, /. Buss. Phys. Chem. 8oc. t 1924, 54, 10 Gessner, Kolloidchem. Beihefte, 1924, 19, 213. 11 Reinders and Van der Zocher, Zeitsch. physical. Chem., 1921, 98, 293 ; compare Lee, Rec. Trav. chim., 1928, 47, 193. COMPOUNDS OF VANADIUM. 61 similar disturbed. A bi-refringence in sols of several slightly soluble substances which form ordinarily microscopic crystals, e.g. mercurous

Cl , and lead PbI has been chloride, Hg 2 2 iodide, 2 , demonstrated by 1 Reinders. Freundlich does not agree, however, that these rod-like structures are for the double necessarily responsible refraction ; he attributes the chief cause to the presence of aggregates of amicronic non-spherical particles, not discernible under the ultramicroscope and 2 separated from one another by amicronic distances. It is adduced in support of this view that double refraction can be produced in and removed from sols of benzopurpurin without any change being visible in the ultramicroscope. Double refraction is also shown, although not 3 so strongly, by aged ferric hydroxide sols, aluminium hydroxide sols, clay suspensions, soap solutions, alizarin, aniline-blue, ^?-azoxyanisole, 4 >-azoxyphenetole. Red gold sols and silver sols also become doubly 5 refracting under the influence of an alternating current. Hydrates of Vanadium Pentoxide. Several hydrates of vanadium pentoxidc have been obtained by the action of mineral acids on solutions of alkali vanadates. In many cases their composition agrees with that of one of the free acids corresponding to the vanadates, but it has been shown that are not true acids 6 the they ; amount of water present 7 8 depends only on the conditions of drying. Von Hauer and Fritzsche both obtained an insoluble dihydrate, V 2O 5.2H 2O, which was supposed to free . be pyrovanadic acid, H 4V 2O 7 By continuing the drying of 9 this hydrate over concentrated sulphuric acid von Hauer obtained the monohydrate, V 2O 5 .H 2O, which similarly was supposed to be free 10 metavanadic acid, HVO 3 . Ditte obtained several other hydrates, V 2O 5 .8H 2O, V 2O5 .2H 2O, V 2O5 .H 2O, the composition of which also depended on the vapour-pressure of the atmosphere. A hydrate corresponding to orthovanadic acid, H 3VO 4 or V 2O5.3H 2O, has not been obtained. When hydrated vanadium pentoxide is precipitated under special conditions it forms a yellow or orange substance known as vanadium Bronze. Boiling a solution of sulphur dioxide with copper vanadate 11 gives scales of a golden or orange colour. The precipitate obtained may, however, be a partial reduction product of the vanadate, in which 12 case its composition would be analogous to that of the tungsten bronzes. A vanadium bronze is also obtained when a solution of ammonium metavanadate is added to a solution of copper sulphate in excess of ammonium chloride until a permanent precipitate forms, the suspension then being heated to 75 C. The more slowly the precipitation takes

1 Beinders, Proc. K. AJcad. Wetensch. Amsterdam, 1916, 19, 189. 2 Freundlich, Colloid and Capillary Chemistry, translated by Hatfield (Methuen, London, 1926), p. 823. 3 Aschenbrenner, Zeitsch. physical. Chem., 1927, 127, 415. * Zeitech. Zocher, loc. cit. ; Freundlich, Schuster, and Zoeher, physical. Chem., 1923,

* 137 Phil Bergholm and Bjornstahl, Physical. Zeitsch., 1920, 21, ; Hag., 1921, [vi], 42, 352. 6 Dullberg, Zeitsch. physical. Chem., 1903, 45, 175. 8 7 von Hauer, J. prakt. Chem., 1860, 80, 324. Fritzsche, ibid., 1851, 53, 93. 9 See also Manasse, Chem. Zentr., 1886, 773. 10 Ditte, Compt. rend., 1885, 101, 698. 11 Bull. Soc. Gerland, J. prakt. Chem., 1871, [ii], 4, 139; chim., 1873, [ii], 19, 501; Ber., 1877, 10, 1515. 12 See this series, Vol. VII., Part III. (1926), p. 243. 62 VANADIUM, NIOBIUM, AND TANTALUM. l states place the more brilliant is the colour of the bronze. Guyard that the bronze is not a hydrate of vanadium pentoxide, but an acid ammonium vanadate. THE VANADAXES,

General. It seems certain that the free acids corresponding to these salts do not exist in the solid state, and that, with the possible are also exception of hexavanadic acid, mentioned below, they incapable acids are known. The of existing in solution, although salts of all the most stable class of salts is the metavanadates, the next in order of the orthovanadates are few stability being the pyrovanadates, while the in number and undergo rapid hydrolysis even in the cold, to give pyro-salts : 2Na 3V0 4 +H 2O^Na 4V 2O 7 +2NaOH ;

the meta-salt on the solution : the pyro-salt is converted into boiling

Na 4V 2 7 +H 2O^2NaVO 3 +2NaOH.

These reactions are reversible, and removal of the caustic alkali by addition of acids effects the immediate conversion of ortho- or pyro-salts into the meta-salts. On the other hand, the presence of a large excess of caustic alkali favours the formation of the ortho- and pyro-salts. The order of stability is the reverse of that which applies to the ortho-, are from the pyro-, and meta-phosphates. Orthophosphates prepared other two classes either by boiling or by addition of weak acids. Metavanadates of the alkalis are white or colourless, and give colourless aqueous solutions which rapidly become yellow, and, on addition of acids, red or orange. These coloured solutions contain polyvanadates, the formation of which is comparable to that of the polychromates and other salts formed by condensation of weakly acid oxides of metals, under definite conditions of tem- e.g. molybdates and borates. Thus, is converted into perature and concentration, potassium nietavanadate in accordance with the : the acid salt 2K 20.3V 2 5 , equation 0. 6KV0 3 +2HC1=2K 20.3V 2 5 +2KC1+H 2 White. Red. 0. Cf. 2K 2Cr0 4 +2HCl=K 20.2Cr0 3 +2KCl+H 2 Yellow. Ked.

Polyvanadates produced in this manner are much more numerous than the R/ 0.nV O the polychromates, and have general composition 2 2 5 , where n is greater than one. Solutions of polyvanadates contain equilibrium mixtures the compositions of which vary considerably with the acidity, the temperature, and the concentration of the solution. Thus, by acidifying a solution of lithium metavanadate with acetic 3Li .12H are acid, crystals having the composition 20.4V 2 5 2 obtained, the mother-liquor from which, on being boiled, gives the compound a solution of 3Li 20.5V 2O5 .14H 20. Again, by treating potassium metavanadate with acetic acid under different conditions of temperature and concentration, a series of salts has been obtained in which and 3. In several salts the molecular of vanadium n =, f , 2, , proportions pentoxide ~and basic oxide are not so simple, for example in the com-

i Guyard, Bull Soc. chim., 1876, pi], 25, 356. COMPOUNDS OF VANADIUM. 63

it is pound 22K 2O.24

(i) 2VO4'"+2IT *V 2O 7 ""+2H 2O (pyrovanadate). (ii) 3V 2 7 ""+6H' ^2V 3 9 '"+3H 2 (metavanadate). (iii) 2V 3O9 '"+3IT ^HV6 O 17 "'+H 2O (hexavanadate). Hexavanadic acid is also stated to be formed in solution by the 2 decomposition of pervanadic acid, which is produced when vanadium pentoxide is treated with hydrogen peroxide, but more recently the properties of the solution have been attributed to the formation of to peroxyorthovanadic acid, HVO 4.H aO (see p. 91). According .9H Diillberg the compound which has the composition Na 2O.2V 2O 5 2O should be formulated as the trisodium salt of hexavanadic acid, which has the com- Na 3HV6O17 .13H 2O ; similarly the compound as the disodium position Na 2O.3V 2O5 .3H 2O should be formulated interest salt of hexavanadic acid, Na 2H 2V6O 17 .2H 2O. It is of some to note that many of the heteropoly-acid compounds which contain 3 vanadium can also be written as being derived from hexavanadic acid, 4 although this theory of their constitution is not now held. The hexavanadic acid theory does not exclude the possibility of the existence of other more highly condensed acids. Many of the alkali polyvanadates can be prepared in two crystalline forms : (a) Orange, transparent lustre. It is crystals, and (b) golden, scaly masses, with a metallic suggested that the latter are derived from the more highly condensed acids. All the vanadates are powerful oxidising agents and undergo reduction in acid solution in the manner already described for vanadium pentoxide. The alkali vanadates are usually easily soluble in water, and are white or pale yellow, crystalline compounds, and frequently

1 Diillberg, Zeitsch. physical. Chem., 1903, 45, 175; compare Bosenheim and Yang, Zeitsch. anorg. Chem., 1923, 129, 181. 2 Pissarjewsky, Zeitsch. physical. Chem.. 1903 43, 173. 3 Prandtl, Zeitsch. anorg. Chem., 1911, 73, 223 ; 1915, 92, 198. 4 Meyer and Pawletta, Zeitsch. anal Chem., 1926, 69, 15* 64 VANADIUM, NIOBIUM, AND TANTALUM. dimorphous. They are insoluble in alcohol, which is often employed to throw them out from solution. When ammonia gas is passed over the vanadates of the heavy metals, or when the latter are digested in liquid ammonia, slow absorption ensues with formation of hexammine addition six molecules of ammonia are taken compounds ; usually up 1 for each atom of metal present.

Orthovanadates, R' 3V0 4 or 3R' 2O.V 2O 5 . The alkali orthovanadates can be prepared by fusing a mixture of vanadium pentoxide with the calculated quantity of alkali carbonate. The best known is the sodium salt Na 3V0 4.12H 20. Ammonium orthovanadate does not 2 appear to exist. Ditte obtained a number of vanadates by fusing a mixture of vanadium pentoxide, sodium halide, and the halide of the desired metal, and this method has furnished several orthovanadates. Copper, lead and silver orthovanadates have been prepared by double decomposition in solution, but addition of metallic salts to a soluble orthovanadate most often yields variously coloured precipitates of uncertain because of their to composition ; strong tendency undergo hydrolysis the ortho-salts give rise to the corresponding pyro- and meta- or to a mixture of them the of the metal vanadates, ; hydroxide may also be produced, and in the case of ferrous sulphate and other reducing agents oxidation takes place. Silver orthovanadate reacts with alkyl halides to give esters of acid. These are of formula vanadic yellow liquids general Alk. 3VO 4, in which Alk. represents the alkyl radical. Esters of orthovanadic acid are more stable than those of and meta-vanadic acids pyro- ; that is, the order of stability is the reverse of that which applies to the 3 inorganic salts. The following orthovanadates have been prepared : Orthovanadate BiVO is a Bismuth , 4 , bright yellow compound obtained by the double decomposition of bismuth nitrate with an alkali vanadate.4 has been as Cerous Orthovanadate, CeVO 4, prepared dark red needles 5 by fusing sodium metavanadate with cerous chloride.

Copper Orthovanadate, Cu 3 (V0 4 ) 2 . When treated with solutions of copper salts, solutions of orthovanadates yield a greenish-yellow precipitate which consists mainly of copper orthovanadate with excess 6 of vanadium pentoxide. Lead Pb is as a white Orthovanadate, 3 (V0 4 ) 2 , precipitated powder when sodium orthovanadate is treated with lead acetate solution. 7

Lithium Orthovanadate, Li 3V0 4 . The yellow, anhydrous salt results on fusing a mixture of lithium carbonate and vanadium pentoxide in 8 the requisite proportions. Crystals of the hetuahydrate, Li 3V0 4.6H 2O, have been obtained by concentrating the mother-liquor left from 9 crystallisation of lithium pyrovanadate. Two other lithium vanadates, 10 4Li 2(XV 2O5 .H 2O and 4Li 2O.V 2 5 .14H 2O, have also been prepared. Ni Nickel Orthovanadate, 3 (V0 4 ) 2 , yields green, prismatic needles

1 Ephraim and Beck, Hdo. Chim. Ada, 1926, 9, 38. 2 Ditte, Corrupt, rend., 1883, 96, 1048. 3 /. Hall, Chem. floe., 1887, 51, 751 ; Hess, J. JSoc. CJiem. Ind., 1914, 33, 712. 4 Carnot, Compt. rend., 1887, 105, 119 ; Erenzel, Jalirb. Miner., 1875, 680. 5 Handbuch der Abegg, anorganischen Chemie (Leipzig), 1906, 3, i, 208. 6 Gerland, Ber., 1876, 9, 873. 7 J. Roscoe, Chem. Soc., 1871, 24, 23 ; Amadori, ibid., Abs., 1919, 116, ii, 413. 8 Bammelsberg, Wied. Annalen, 1883, [ii], 20, 928. 9 1168. " Ditte, Compt. rend., 1887, 104, Ditte, loc. cit. COMPOUNDS OF VANADIUM. 65

when a fused mixture of vanadium pentoxide, sodium bromide and a small proportion of nickel bromide is extracted with dilute nitric acid and the solution concentrated. 1 as a Potassium Orthovanadate, K 3VO 4, forms pale yellow, crystalline mass when vanadium pentoxide and potassium carbonate are fused 2 together in the necessary molecular proportions. It melts at a tem- 3 perature above 1000 C. A solution of vanadium pentoxide in three of caustic potash yields colourless, transparent, equivalent proportions T deliquescent crystals of two hydrates, 2K 3VO 4 .9H 2O and K3\ O 4.6H 2O 4 respectively, according to the temperature of crystallisation. If a large excess of caustic potash is employed the compound obtained is

is as a Silver Orthovanadate, Ag 3VO 4 , precipitated deep orange powder when a freshly prepared solution of sodium orthovanadate is 6 treated with a carefully neutralised solution of silver nitrate. It melts 7 between 408 and 565 C., and is soluble in nitric acid and in ammonium the latter solution of the hydroxide ; yields yellow hexagonal crystals O.8 composition 3AgVO 3 .2NH 3.H 2 Sodium Orthovanadate, Na 3VO 4.12H 2O, is the orthovanadate most frequently met with. It is readily obtained by adding excess of caustic soda to a solution of sodium pyrovanadate :

It can be conveniently crystallised from caustic soda solutions, in which it is less soluble than in water. It forms hexagonal prisms isomorphous .12H and with the corresponding phosphate and arsenate, Na 3PO 4 2 .10H Na AsO .10H Na 3As0 4.12H 2O. The decahydrates, Na 3V0 4 20, 3 4 2O, 9 In of the and Na 3PO 4.10H 20, are also isomorphous. consequence reversible nature of the above reaction, solutions of sodium ortho- their electrical has been vanadate are strongly alkaline ; conductivity 10 melts at 866 n studied by Dtillberg. The anhydrous salt, Na 3VO 4 , or 850 C.12 It can be prepared by fusing sodium carbonate and Extraction vanadium pentoxide in the required molecular proportions. colourless of the product with water and precipitation with alcohol gives sixteen molecules of needle-shaped crystals of a hydrate containing 13 and Na VO .TH 2O water, Na 3VO 4.16H 2O. The hepta- octa-hydrates, 3 4 also been 14 and Na 3VO 4.8H 2O, have prepared. in a excess of caustic soda, By dissolving vanadium pentoxide large 15 contained a Ditte obtained two crystalline vanadates which larger pro- the and to portion of the basic oxide than is present in orthovanadate, .30H O and 4Na O.V .26H O. which he gave the formulae 4Na 2O.V 2 5 2 a 2 5 2 is obtained in Strontium Orthovanadate, Sr 3 (VO 4 ) 2, transparent, pale of vanadium yellow leaves by heating together a mixture pentoxide, 16 sodium iodide, and strontium iodide. . is a brown substance obtained Thallium Orthovanadate, T1 3V0 4, light of thallium carbonate by carefully fusing three molecular proportions 2 loc. ctt. i Ditte, Compt. rend., 1883, 96, 1048. Rammelsberg, 3 * 902, 1061. Oanneri, G/zzetta, 1928, 58, 6. Ditte, Compt. rend., 1887, 104, * * ' J. Ckem. 1878, 33> 273. Ditte, loc. cit. Roscoe, loc. cit. Carnelly, ^oc., 8 9 J. Chem. Soc., 47, 357. Ditte, Compt. rend., 1887, 104, 1705. Baker, ^1885, 1 and toe. <**. Zeitsch. Chem., 1903, 45, 129 ; Rosenheim Tang, Diillberg, physikal. 1S i* 12 cit. Roscoe, loc. ctt. Carnelly, loc. cit. Canneri, loc. " J. Chem. 94. Baker, loc. cit. ; cf. Hall, Soc., 1887, 51, " 16 1883, 96, 1048. Ditte, Compt. rend., 1887, 104, 902, 1061. Ditte, ibid^ ** VOL. vi. : in. 66 VANADIUM, OTOBIUM, AOT) TANTALUM. with one molecular proportion of vanadium pentoxide. Its density is 1 a 8-6. One gram dissolves in a litre of water at 15 C. M.pt. 566 or 555 C.3 Double Compounds of Orthovanadates and Halogen Salts. Although the orthovanadates of divalent metals are not very stable compounds, they form very stable double salts with certain metallic halides. These double salts are of interest in that isomorphous analogues are frequently given by orthophosphates and orthoarsenates. The following 4 naturally occurring double salts are isomorphous : .PbCl 3Pb 3 (V0 4 ) 2 2 (vanadinite). .PbCl 3Pb 3 (P0 4 ) 2 2 (pyromorphite).

3Pb 3 (As0 4 ) 2 .PbCl 2 (mimetesite).

They are isomorphous also with the calcium compounds Ca 3 (V0 4 ) 2 . 5 .CaCl . of CaCl 2 and Ca 3 (P0 4 ) 2 2 The discovery isomorphism among these double salts led Roscoe to transfer vanadium from the chromium family, in which it had been previously placed, to Group V. (see p. 24). The general method of preparation of these double salts consists in fusing vanadium pentoxide with excess of the halide of the metal. Except in the case of the sodium compound, the residue is simply washed with water, in which the double salts are insoluble. The following have been described :

2Na 3V0 4.NaF.19H2 O . Limpid octahedra, isomorphous with the corresponding phosphorus and arsenic compounds. 6 7 3Ba 3 (V0 4 ) 2 .BaBr2 . Hexagonal, transparent plates. 8 .BaI . 3Ba 3 (V0 4 ) 2 2 Hexagonal, brown, transparent prisms.

3Cd .CdCl , 5-264*. 3 (V0 4 ) a 2 Hexagonal prisms ; density

8Cd .CdBr' . 5-456. 8 (VO 4 ) a a Hexagonal prisms ; density 9 (The corresponding cadmium iodide has not been isolated. ) 10 Ca .CaCl . . 3 (V0 4 ) 2 2 Glistening, rhombic crystals. 11 Ca .CaBr . 3 (V0 4 ) 2 2 Glistening, thin leaves. 12 3Ca .CaBr . 3 (V0 4 ) 2 2 White, glistening crystals. 13 3Ca VO .CaI . 3 ( 4 ) 2 2 Hexagonal, colourless, transparent crystals.

3Pb .PbCl . 3 (V0 4 ) 2 2 Reddish-brown, transparent, hexagonal 14 prisms. (This lead salt is the artificial form of the important natural ore vanadinite. ) 15 3Sr .SrBr . 3 (V0 4 ) 2 a Hexagonal plates. 3Sr s (V0 4 ) g.SrV

1 Carnelly, J. Chem. Soc., 1873, 26, 325. 2 3 Carnelly, ibid., 1878, 33, 273. Canneri, loo. ctt. * Roscoe, /. Chem. Soc., 1868, ai, 322. The refractive indices of these ores have been measured by Bowman (Min. Mag., 1903, 13, 324). 6 Hautefeuille, Compt. rend., 1873, 77, 896. 6 J. Chem. Baker, Soc., 1885, 47, 358; Travers, Bull Soc. chim., 1923, [iv], 33, 297. 7 Ditte, Compt. rend., 1883, 96, 575, 846 ; Ber., 1883, 16, 1097. 8 Ditte, foe. cit. de Schulten, Butt. Soc. chim., 1900, 159. 10 p], 23, toe. cit 11 Hautefeuille, Ditte, Compt. rend., 1883, 96, 575. 12 * Ditte, ibid., 1882, 94, 1592 ; 1883, 96, 575, 846, 1226. foe. cit. 11 Ditte, Hautefeuille, foe. cit. ; /. Chem. Roscoe, Soc., 1871, 24, 23 ; Carobbi and Restaino, Gazzettct, 1926, 56, 59. 15 Ditte, Compt. rend., 1883, 96, 575, 1846. is Ditte, foe. cit. COMPOU1TOS OF VANADIUM. 67 Chlororthovanadates of beryllium and zirconium* and chlormetavanadates of lead and have copper (see p. 45), also been prepared.

Sodium Stannovanadates or Vanadostannates. By crystallising mixed solutions of sodium staimate and sodium orthovanadate, a series of sodium Stannovanadates has been the prepared, salts havin& the following compositions : 3Na 3VO 4.Na 2Sn0 3.32H 20. 4Na3V0 4.Na 2Sn0 3.4SH 0. 5Na 2 3V0 4.Na 2Sn0 3 .64H 20. 6Na 3VO 4.Na 2Sn0 3 .80H 20.

The constant difference between each is member Na 3V0 .16H (X The member 4 particular obtained depends on the concentration of the constituents and on the temperature. The same compounds can be prepared by fusing together stannic Sn0 oxide, 2 , vanadium pentoxide and caustic soda, and the crystallising fused mass. They are all iso- and form morphous, doubly refracting, transparent, rhombic needles. The use of sodium orthoarsenate and sodium orthophosphate instead of the vanadate rise to two gives analogous series of stannophosphates and stannoarsenates. A mixed salt having the composition of both the and arsenate phosphate has also been isolated :

4Na .Na 3 (P,V)0 4 2Sn0 3.48H aO. When a solution containing sodium orthovanadate and stannous chloride is carefully neutralised with caustic soda, an amorphous substance is yellow obtained which has the composition

Na 20.4Sn0 2.V 2 5.0H 2 or Na O.3Sn0 .V a 2 2 5.0H 20.

This is considered to be a salt compound complex of a heteropoly-acid formed from stannic oxide and vanadium pentoxide, and it is therefore comparable with the vanadoselenites and vanadotellurites.2

Pyrovanadates, R' V or 2lT . 4 2 7 20.y 2 5 The alkali pyrovanadates are prepared by dissolving the equivalent quantity of vanadium in solutions of or the pentoxide alkalis, by spontaneous decomposition in solution of the alkali orthovanadates. Pyrovanadates of other metals are obtained by fusing vanadium pentoxide with the salts or of the metals in molecular hydroxides proportions, or, when they are double sufficiently insoluble, by decomposition between an alkali pyrovanadate and a salt of the metal required. are Pyrovanadates more stable than orthovanadates, but, in conse- of the acid character of quence weakly pyrovanadic acid, they undergo conversion into the easy metavanadates. Sodium pyrovanadate in solution is thus converted carbon by dioxide into the metavanadate : Na V O 4 2 7 +CO 2 ==2NaV0 3 +Na 2CO 3 .

Ammonium pyrovanadate does not appear to exist; addition of

1 Tanatar and Kucwski, J. Euss. Phys. Chem. 8oc^ 1909, 41, 813. 2 Prandtl and Eosenthal, Ber. 9 1907, 40, 2125. 68 VANADIUM, NIOBIUM, AND TANTALUM. ammonium chloride to a solution of a pyrovanadate precipitates ammonium metavanadate : 0. 4NH 4Cl+Na 4V 2 7 =2NH 4V0 3 +4NaCl+2NH 3 +H 2

These reactions suggest that sodium pyrovanadate undergoes partial hydrolysis in solution :

2NaV0 3 +2NaOH.

The introduction of carbon dioxide into the solution removes the caustic of view of the ionic the OH' soda (or, from the point theory, ions), so that the equilibrium is disturbed and the reaction then proceeds of ionised ammonium completely from left to right. Similarly, addition the ions chloride suppresses the concentration of OH' already present of the therefore in solution ; more pyrovanadate undergoes hydrolysis, in order that the equilibrium concentrations of ions shall be maintained, until all the pyrovanadate is converted into metavanadate. The weakly acid nature of pyrovanadic acid is also shown by the fact that solutions of pyrovanadates react alkaline to phenolphthalein. ion. 1 The solutions have been shown to contain the colourless (V 2 7 )"" Silver pyrovanadate gives esters of pyrovanadic acid only with the 2 V . higher alkyl halides, e.g. amyl pyrovanadate, (C 5Hn ) 4 2 7 The following pyrovanadates have been prepared : Ba is on addition of Barium Pyrovanadate, 2V 2 7 , precipitated barium chloride to a solution of sodium pyrovanadate or of other 3 vanadates in the presence of ammonia. It has more recently been on vanadium 4 prepared by the action of barium peroxide pentoxide. 5 It is a white, amorphous powder which melts above 863 C. is calcium Calcium Pyrovanadate, 2Ca 2V 2 7 .5H 20, formed by adding chloride to a solution of sodium pyrovanadate and drying the precipitate 6 .2H has been obtained in at 100 C. The dihydrate, Ca 2V 2 7 20, to meta- transparent needles by adding calcium chloride ammonium 7 vanadate solution and then excess of ammonium hydroxide. O .3H has been obtained in Cu 3 Copper Pyrovanadate, 2V 2 7 2 greenish- the action of yellow, transparent, rhombohedral plates by copper 8 salt has sulphate on ammonium metavanadate. The anhydrous also been prepared by saturating a solution of vanadium pentoxide 9 10 and a copper salt with ammonia. According to Radau, addition of copper sulphate to solutions df pyrovanadates gives rise to pre- of which to the formula cipitates the composition approximates

8Cu0.3V 2O5 .

. This salt is of interest in that it is Lead Pyrovanadate, Pb 2V 2O 7 the artificial form of desdoizite, one of the important natural ores of vanadium. It is obtained by boiling mixed solutions of lead nitrate 11 and ammonium metavanadate in the presence of acetic acid. A of 2Pb .Pb0 is pale yellow basic pyrovanadate lead, 2V 2O7 5 obtained by

1 Dullberg, Zeitsch. physikal Chem., 1903, 45, 129. 2 Hall, J. Chem. Soc., 1887, 51, 751. 3 Boscoe, ibid., 1871, 24, 23 ; Carnot, Compt. rend., 1887, 104, 1803, 1850. 4 Hedvall and Zweigbergk, Zeitsch. anorg. Chem., 1919, 108, 119. 6 s Carnelly, J. Chem. Soc., 1878, 33, 273. Roseoe, loc. cit. 8 7 Ditte, Compt. rend., 1887, 104, 1705. Bitte, loc. cit. 9 Carnot, Compt. rend., 1887, 105, 121. ll Radau, Dissertation (Berlin, 1888). Ditte, kc. cit. COMPOUNDS OF VANADIUM. 69 the addition of lead acetate to a solution of a vanadate. The com- in this manner is, however, not position of the compound produced 1 constant, . ,. Li O When a solution of sodium Lithium Pyrovanadate, 4V 2 7 .6H2O. and metavanadate is made strongly alkaline with lithium hydroxide are concentrated in vacuo, white, silky needles of the hexahydrate the obtained, which leave a white, nacreous mass of anhydrous salt, 2 been heated. The Li V aO .4H 2O, has Li V O , on being tefrahydrate, 4 ? 4 2 7 and obtained vanadium pentoxide with lithium nitrate by fusing 3 in which it is soluble. extracting the melt with water, readily in brilliant Mn V , is obtained large, Manganese Pyrovanadate, 2 2 7 of vanadium brown needles by fusing together a mixture pentoxide, 4 sodium bromide and manganese bromide. .4H out in colourless, Potassium Pyrovanadate, K 4V 8 7 aO, separates one molecular of vanadium transparent crystals when proportion molecular of pentoxide is dissolved in two proportions potassium and leave a hydroxide These lose water when heated, melt, crystalline salt V O 5 which is and melts mass of the anhydrous K 4 2 7 , deliquescent 6 K V O . at 910 C. A white, crystalline, readily soluble trihydrate, 4 2 7 7 3ELO, has also been prepared. is obtained as a dense yellow pre- Silver Pyrovanadate, Ag4V 2 7 , of neutral silver nitrate to sodium pyro- cipitate by the addition vanadate solution.8 It has also been obtained as brilliant yellow, 9 383 C.10 transparent plates. M.pt. , u * .18H is fusing Sodium Pyrovanadate, Na4V2 7 20, prepared by sodium carbonate mols.) and vanadium pentoxide (1 mol.) with (2 the vanadium pent- extracting the melt with water. Alternatively, the same molecular oxide is dissolved in caustic soda solution, using or needles proportions. It forms long, six-sided plates, pearly, glistening At 100 C. it loses which are efflorescent and readily soluble in water. evolved at 140 L. seventeen molecules of water, the last molecule being u 12 The anhydrous salt melts at 654 or 632 C. By partial dehydration, a mixture of alcohol and water, an octahydrate, and crystallisation from has been obtained.13 Na 4V 2O 7 .8H 20, T1 V O is as a light yellow Thallium Pyrovanadate, 4 2 7 > precipitated a saturated powder by the addition of thallium sulphate to cold, on a mixture solution of sodium orthovanadate. It also results fusing carbonate. 14 It melts at 4,54 or of vanadium pentoxide and thallium 16 5000 of water at 14 U 416 C., and is soluble in about parts 78; Amadori, 1 oft. Ann. Ohm. Phys,, 1854, [iii], 41, Roscoe, toe. ; Descloiseaux, 413. J. Ohem. Soc., Aba., 1919? 116, ii, 2 Ditto, Compt. rend., 1887, 104, 11C8. 928. Rammelsberg, Wud. Annoten, 1883, [ii], 20, Ditte, Compt. rend., 1883, 96, 1048. Ditto, ibid., 1887, 104, 902. Canned, Gazzetta, 1928, 58, 6. Rammelsberg, toe. cit. Roscoe, loc. cit. Ditte, Compt. rend., 1887, 104, 1705. 273, Carnelly, J. Ohem. 8oe., 1878, 33, " Carnelly, loc. cit 12 Canneri, loc. cit. Ditte, Gompt. rend., 1887, 104, 1061. 14 323. Carnelly, J. Ohem. Soc., 1873, 26, 5 1 Carnelly, #&, 1878, 33 273. 16 Canneri, loc. cf. 70 VANADIUM, NIOBIUM, AND TANTALUM.

is and is Thorium Pyrovanadate, Th 2V 2O 7 .6H 20, greenish-yellow, obtained by the action of thorium chloride in dilute solution on ammonium mctavanadate. 1 forms when vanadium Zinc Pyrovanadate, Zn 2V 2 7 , orange-red prisms 2 pentoxide is fused with a mixture of sodium bromide and zinc bromide. It is appreciably soluble in water. Several vaiiadates are known the compositions of which are inter- the 2R' O.V and that of mediate between that of pyrovanadates, 2 2O5 , the metavanadates, R' 2O.V 2O5 . They can be looked upon as basic metavanadates. The following have been described :

Basic Lithium Metavanadate, 3Li 20.2V 2 5.15H 2 or Li 20.4LiV0 3 . dilute 15H 20, is obtained by crystallising lithium orthovanadate from nitric* 9,cid ^

Basic Potassium Metavanadate, 5K 20.4V 2 5.7H 2 or K 20.8E:V0 3 . in white when a solution of 7H 2 5 separates crystals potassium pyro- 4 is acetic vanadate, K 4V 2 7 , acidified with acid and concentrated.

Basic Sodium Metavanadate, 3Na 2O.2V 2 5 .2H 2 or Na 2O.4NaVO 3 . 2H 20, and the hexahydrate, Na 20.4NaV0 8.6H 20, have both been prepared by fusing sodium carbonate (3 mols,) with vanadium pent- 5 oxide (2 mols.). They are not readily soluble in water. is Basic Thallium Metavanadates, 3T1 20.2V 2 5 or T1 20.4T1VO S , obtained as a sparingly soluble, yellow powder by the action of thallium 6 sulphate on the corresponding sodium compound. is 6T1 2O.5V 2 5 or T1 20.10T1VO 3 prepared by the action of thallium in the of sulphate on sodium pyrovanadate, Na4V 2 7 , presence excess 7 of vanadium pentoxide. It dissolves in 9372 parts of water at 11 C. and in 33G6 parts of water at 100 C. Silver or is Basic Metavanadate, 8Ag 20.2V 2 5 Ag 20.4AgVO 39 prepared by the action of silver nitrate on the corresponding sodium salt. It is a dark yellow compound, almost insoluble in water.8

Basic Lead .2H or . Metavanadate, 3PbO.2V 2O 5 2 PbO.2Pb(V0 3 ) 2 2H 20, is obtained as a yellow powder by the action of lead nitrate on an acid manganese vanadate. 9

Basic Strontium or . Metavanadate, 3Sr0.2V 2 5 .2H 2 Sr0.2Sr(V0 3 ) 2 10 2H 2O, has also been prepared.

Metavanadates, R/V0 3 or R' 2O.V 2 5 . These salts are more stable than either the ortho- or pyro-vanadates. Solutions of the latter yield metavanadates on being evaporated or by treatment with carbon dioxide. The alkali metavanadates are prepared directly by dissolving vanadium pentoxide in the calculated quantity of alkali hydroxide. The metavanadates of other metals are prepared by fusing vanadium pentoxide with the oxide or carbonate of the metal in 11 calculated quantity, or by the action of a soluble salt of the metal 1 Volck, Zeitech. anorg. Ohem., 1894, 6, 161. 2 Ditte, Compt. rend., 1883, 96, 1048 ; c/. Radau, Annakn, 1889, 251, 114. 3 Rammelsberg, Wied. Annakn, 1883, pi], 20, 938. 4 Rammelsberg, he. cit. 6 Carnelly, Annalen, 1873, 166, 155. 6 Carnelly, J. Chem. Soc., 1873, 26, 330. 7 Carnelly, loc. c^t. 8 Carnelly, loc. cit. 9 Ephraim and Beck, Helv. Ghim. Acfa, 1926, 9, 38. 10 Ephraiin and Beck, loc. cit 11 de Carli, Atti R. Accad. Lincei, 1925, [vi], i, 533." COMPOUNDS OF VANADIUM. 71

on a neutral solution of alkali metavanadate. The alkali and alkaline earth metavanadates are white or pale yellow ; metavanadates of the metals are or heavy deep yellow, brown, red. Most of the alkali metavanadates are soluble in water ; vanadates of the heavy metals are Almost insoluble in water or dilute acetic acid. According to Bleecker,1 11 the vanadates of the metals seem to be soluble in water to some xtent, but become insoluble in the presence of small quantities of the irecipitating agent. They show a tendency to precipitate in the olloidal state; especially is this the case with the vanadates of iron, opper, zinc and aluminium. The vanadates of mercury, lead, copper nd iron fuse at about 600 C., but the vanadates of aluminium, calcium, ^inc and tungsten do not fuse at much higher temperatures. The fused vanadates of iron and copper are extremely hard and are good conductors of electricity. On being treated with mineral acid, vanadates decompose with the formation of red colloidal vanadium pentoxide. The following metavanadates have been prepared : Aluminium be Metavanadate, A1(VO 3 ) 3 , may prepared electrolytically or by the addition of an aluminium salt in solution to an alkali vanadate. 2 is Ammonium Metavanadate, NH 4VO 3 , one of the commonest compounds of vanadium, and forms the starting material for the preparation of a large number of vanadium salts. Its preparation from a vanadium ore has been described on p. 54. It is obtained in the laboratory by dissolving vanadium pentoxide in excess of ammonium hydroxide and concentrating, or, instead of concentrating, alcohol may be added, in which the salt is insoluble. Recrystallisation from dilute 3 ammonium hydroxide gives a pure product. Ammonium metavanadate is also insoluble in a saturated solution of ammonium chloride, and it is quantitatively precipitated by the addition of an excess of solid ammonium chloride to a neutral solution of sodium metavanadate or 4 pyrovanadate. It is a white powder which can be obtained in colourless, granular crystals, isomorphous with potassium metavanadate. Its density is 2-326. One hundred parts of water dissolve 5-18 parts at 15 C. 5 and 10-4 parts at 32 C. On being gently heated in air ammonium metavanadate becomes yellow, red, and then brown, with loss of e ammonia :

2NH 4V0 3 (solid)=V 2 5 (solid)+2NH 3 (gas)+H 2O (liquid) 43,600 calories. At temperatures above 210 C. the salt undergoes reduction and leaves a residue of the lower oxides of vanadium, and may also give some nitride. 7 .H is obtained in white or Barium Metavanadate, Ba(VO 3 ) 2 2O, yellow microscopic crystals by the action of barium chloride on

1 Bleeeker, Met. Chem. Eng. 9 1910, 8, 666. 2 Bleecker, ibid., 1911, 9, 501. 3 Lackartre, Bull 8oc. chim., 1924, [iv], 35, 321. * Gooch. and Gilbert, Zeitsch. anorg. Chem., 1902, 32, 174 ; Rosenheim, ibid., 1902, Ber. 3164. 32, 181 ; Campagne, 9 1903, 36, 5 loc. cit. 918 : Lackartre, ; compare, however, Ditte, Compt. rend., 1886, 102, Meyer, Zeitsch. EleUrochem., 1909, 15, 266. 6 Matignon, Chem. Zeit., 1905, 29, 987. 7 698. Roseoe, Annakn, SuppL, 1868, 6, 104 ; Ditte, Compt. rend., 1885, 101, 72 VANADIUM, NIOBIUM, AND TANTALUM. 1 between 190 potassium metavanadate. It undergoes dehydration and 200 C. The anhydrous salt has also been prepared by the action 3 of barium peroxide on vanadium pentoxide. .4H Addition solutions Beryllium Metavanadate, Be(V0 3 ) 2 2 ^of to basic vanadates of beryllium salts to alkali vanadates gives rise is obtained of indefinite composition. The pure salt by boiling in water in the beryllium hydroxide and vanadium pentoxide required concentrated to a and proportions. The solution is filtered, syrup modified an octa- poured into alcohol, whereupon isometric cubes, by hedron, are obtained. The larger crystals polarise light. Density, 3 2-273. One gram dissolves in a litre of water at 25 C. is obtained as brilliant, trans- Cadmium Metavanadate, Cd(VO 3 ) 2 , a mixture of vanadium parent, slender, yellowish needles by fusing bromide.4 pentoxide, sodium bromide and cadmium obtained Caesium Metavanadate, CsVO 3 , has been by boiling 5 vanadium pentoxide with ccesium carbonate solution. .3H rise to Calcium Metavanadate, Ca(VO 3 ) 2 2O, gives bright yellow needles when a solution of ammonium metavanadate is boiled with 6 tetra- calcium chloride solution and precipitated with alcohol. The is a mixture of 3 ) 2 .4H 2O, prepared by allowing potassium hydrate, Ca(VO 7 metavanadate and calcium chloride to evaporate for several days. which Anhydrous calcium metavanadate is a white, porous substance, acids to is unaffected by strong heating, but is readily decomposed by 8 is more soluble in water than yield vanadium pentoxide. It much strontium metavanadate. out on a Cobalt Metavanadate, Co(V0 3 ) 2.3H 20, separates boiling solution of ammonium vanadate with excess of cobalt nitrate which 9 water. has been feebly acidified with nitric acid. It is easily soluble in Copper Metavanadate. Addition of copper sulphate solution to sodium metavanadate throws down a precipitate which consists mainly of copper metavanadate, which is light yellow. The precipitate may, however, be green or blue, because its composition varies considerably. Copper metavanadate can also be produced electrolytically. On being fused at a high temperature in a graphite crucible it forms copper and vanadium carbide. 10 " " is thrown down as a fine, Didymium Metavanadate, "Di"V0 8 , when ammonium metavanadate is treated with grey" precipitate" didymium nitrate.11 .2H is a substance Indium Metavanadate, In(V0 3 ) 3 20, yellow produced 12 by the action of sodium metavanadate on indium chloride solution. 1 Manasse, Annalen, 1887, 240, 23. 2 Hedvall and Zweigbergk, Zeitsch. anorg. Chem., 1919, 108, 119. 3 J. Russ. Brinton, /. Ataer. Chem. Soc., 1916, 38, 2361 ; Tanatar and Kurowski, Soc. Phys. Chem. t 1909, 41, 813. * Ditto, Compt. rend., 1883, 96, 1048. 6 Chabrie, Ann. Chim. Phys., 1902, [vii], 26, 228. 8 Scheuer, Zeitsch. anorg. Ckem., 1898, 16, 284. 7 Manasse, Anmlen, 1887, 240, 44. 8 Bleeoker, Met. Chem. Eng., 1910, 8, 666. 9 Ditto, Compt. rend., 1887, 104, 1705 ; cf. Carnot, ibid., 1887, 105, 119, and Hedvall, Zeitsch. anorg. Chem., 1915, 93, 313.

Bleeoker, Met. Chem. Sng., 1910, 8, 666 j 1911, 9, 501. 11 " " Cleve, Bull. Soc. chim., 1885, 43, 359. Didymium is now known to consist of two v. elements, namely, praseodymium and neodyrnium ; see Auer Welsbach, Monatsh., 1885, a, 477. 12 Benz, Dissertation (Breslau, 1902). COMPOUNDS OF VANADIUM. 73

Iron Vanadate is, metallurgically, the most important vanadate. of a Precipitation solution of a vanadate with ferrous sulphate gives rise to a precipitate of indefinite composition, ortho-, pyro-, meta-, and perhaps a poly-vanadate being present, as well as ferric or ferrous oxide. Reduction of the vanadate to a vanadyl salt may also ensue. The precipitate is usually colloidal and carries down with it some sodium vanadate. The dried powder may be either green, yellow, brown, or red the more the ; nearly precipitate approximates to a red colour the lower is its vanadium content. An iron vanadate has also been prepared by electrolysis of a solution of sodium vanadate between iron 1 poles.

Lead . solutions Metavanadate, Pb(VO 3 ) 2 Acid of vanadates on being treated with lead salts give rise to yellow basic vanadates the composition of which varies with the conditions. The precipitation of normal lead vanadate is, therefore, difficult. It has been accom- plished by the addition of lead acetate to ammonium metavanadate 2 solution in the presence of acetic acid. The lead precipitates contain all the vanadic acid originally present in solution, and precipitation of vanadates with lead salts has, therefore, been employed for the quantitative estimation of vanadium.3 The mineral deschenite consists of lead a chiefly metavanadate ; portion of the lead is, however, fre- 4 quently replaced by zinc. Lithium Metavanadate, LiVO 3.2H 2O, forms brilliant, silky needles when lithium carbonate (1 mol.) and vanadium pentoxide (1 moL) are boiled together in water and the product concentrated in a vacuum. 5 6 It melts at 618 C. and is readily soluble in water. .6H is as trans- Magnesium Metavanadate, Mg(V0 3 ) 2 2O, obtained parent crystals by boiling an excess of basic magnesium carbonate with vanadium pentoxide and concentrating the filtered solution in a 7 vacuum. It is easily soluble in water. is a Manganese Metavanadate, Mn(V0 3 ) 2.4H 2O, sparingly soluble, dark red powder, obtained by interaction between manganese sulphate and ammonium metavanadate in solution. When the powder is boiled in the precipitating solution, reddish-brown six-sided plates of the are 8 anhydrous metavanadate, Mn(VO 3 ) 2 , produced. is as an Mercurous Metavanadate, HgVO 3, thrown down orange precipitate when mercurous nitrate is added to a solution of a vanadate. If the solutions are carefully neutralised the precipitation is complete, and it is used for the gravimetric estimation of vanadium, the mercurous vanadate being ignited and the residue weighed as vanadium pentoxide. In the presence of a slight excess of ammonia, a grey or black precipitate of complex composition is produced. Addition of mercuric chloride to a neutral solution of a vanadate produces a white precipitate, soluble 9 a is thrown down. in acids ; in the presence of ammonia yellow compound

1 Bleecker, loc. cit. 2 Bleecker, loc. cit. 3 Roscoe, Annalen, SuppL, 1872, 8, 102. 4 Bergemann, Pogg. Annalen, 1850, 80, 393. 5 Ditte, Compt. rend., 1887, 104, 1168; Rammelsberg, Wied. Annalen, 1883, pi], 20, 938. 6 Canneri, Gazzetta, 1928, 58, 6. 7 48. Ditto, Compt. rend., 1887, 104, 1705 ; Manasse, Annalen, 1887, 240, 8 284. Radau, Ohem. Zenfr., 1888, 1378 ; Scheuer, Zeitsch. anorg. Ohem., 1898, 16, * Carnot, loc. cit. 74 VANADIUM, OTOBItJM, AM) TANTALUM.

Nickel as trans- Metavanadate, Ni(VO 3 ) 2 , separates greenish-yellow, parent prisms when a solution of ammonium metavanadate is boiled 1 with excess of nickel nitrate solution feebly acidified with nitric acid. is as white or colourless Potassium Metavanadate, KV0 3 , obtained crystals by dissolving vanadium pentoxide in hot, strong caustic potash solution. By varying the concentrations several hydrates have also

: .2II .5H . been prepared 2KV0 3.3H 2 ; KV0 3 2 ; 2KV0 3 2 ; KVO 3 2 leave 3H 2O. On being heated, all these hydrates lose their water and 3 a white, nacreous mass of the anhydrous salt, which melts at 495 C. 4 Two other hydrates were prepared by Rammelsberg : KV0 3 .H 2O and KV0 3.7H 20. Silver is a substance Metavanadate, AgV0 3 , yellow, gelatinous obtained by interaction between silver nitrate and a metavanadate in 5 neutral solution. It is soluble in nitric acid and in ammonia. is Sodium Metavanadate, NaV0 3 , manufactured industrially by decomposing commercial iron vauadate with sodium carbonate or sodium hydroxide, or by dissolving vanadium pentoxide in solutions of these sodium compounds. It was prepared in the pure state 6 by McAdam, to determine the atomic weight of vanadium. Its 7 8 electrolytic production has been studied. Ditte reported the

existence of several hydrates : NaV0 3.2H 2 ; NaV0 3.2|H 2 ; NaVO 3 . 9 in 3H 2O or 4H 2O ; but solubility experiments indicate the existence solution of only the dihydrate, NaV0 3 .2H 20. Cryoscopic measurements show that the anhydrous salt associates in solution to give 10 Na 3V 3 9 . One hundred grams of water dissolve 21*1 grams of anhydrous sodium metavanadate at 25 C. and 38-8 grams at 70 C. The salt possesses such powerful colouring properties that 1 part of it imparts a yellow tint to 200,000 parts of water. The hydrated salt is efflorescent, 11 and melts at 562 C. to a dark red, amorphous mass. The m.pt. of 12 the anhydrous salt is also given as 630 C. It is decomposed by mineral acids in the cold to colloidal give vanadium pentoxide ; when a current of hydrogen chloride gas is passed over sodium metavanadate at 440 C., the whole of the vanadic acid is volatilised and sodium chloride is left. The volatile product condenses as a semi-opaque, is 13 reddish-brown, oily liquid, which probably 2VO 2 .4HC1.3H 20. Strontium is Metavanadate, Sr(V0 3 ) 2.4H 20, obtained as colourless, monoclinic prisms by the action of strontium chloride on potassium metavanadate. It is only sparingly soluble in water and undergoes dehydration at 280 C. 14 Thallium is Metavanadate, T1V0 3 , prepared by fusing thallium

1 Ditto, Compt. rend., 1887, 104, 1705; c/. Carnot, ibid., 1887, 105, 119. 2 Ditte, ibid., 1887, 104, 902. 3 * Canneri, toe. cit. Bammelsberg, loc. cit. 5 Bleecker, loc. cit. ; Browning and Palmer, Amer. J. Sci., 1910, fiv], 30, 220. 6 J. McAdam, Amer. Ghem. Soc., 1910, 32, 1603 ; see ako MeAdam and Picric, ibid., 1912,34,604. 7 Pischer, Trans. Amer. Ehctrochem. Soc. f 1916, 30, 194. 8 Ditte, Compt. rend., 1887, 104, 10C1. 9 McAdam and Pierle, loc. cit. 10 ZeitscL Dullberg, yhysikal. Ckem., 1903, 45, 129 ; but compare Canned, Oazzetta, 1923, 56, ii, 779. 11 Carnelly, /. Gh&m. 8oc., 1878, 33, 273. 12 Canneri, loc. cit. 13 Smith and Hibbs, Zeitech. anorg. Cfam., 1894, 7, 41. 14 Manasse, Anwkn* 1887, 240, 33. COMPOUNDS OF VANADIUM. 75 carbonate with vanadium 1 pentoxide in molecular proportions. It forms dark, laminated crystals, which are almost insoluble in water Its is 6-019 at 17 C. 2 3 sp, gr. ; m.pt. 424 or 391 C. Zinc .2H forms Metavanadate, Zn(VO 3 ) 2 20, brilliant, pale yellow, cubic or rhombic crystals when zinc nitrate is added to a neutral solution of ammonium metavanadate and the mixture is concentrated. 4 5 It has also been obtained electrolytically. Chlormetavanadates of lead and of copper have also been prepared (see p. 45). Metavanadic acid yields addition compounds with hydroxylamine and ammonia^ When hydroxylamine hydrochloride and ammonium metavanadate are added to a cold saturated solution of ammonia and the whole at C. kept until precipitation takes place, rosettes of lemon- the yellow crystals having composition HVO 3.2NH 2OH.2NH 3 or V0 N are formed. These are 5 4H13 , stable in the presence of ammonia, but are decomposed by water, dilute caustic soda, air, and carbon dioxide. On addition of hydrochloric or sulphuric acid, evolution of nitrous oxide takes place. By decreasing the proportion of ammonium metavanadate added to the saturated ammonia solution, yellow of HV0 .3NH or crystals composition 3 2QH.2NH 3 V06N5H 16 separate out. An unstable extremely compound, HVO 3.3NH 3OH or VO6N3H10 , has also been prepared. Addition compounds with hydroxylamine are also given by arsenic acid and phosphoric acid, and by tungstates, 7 uranates, and molybdates. A reaction which is very comparable to that which takes place with hydroxylamine consists in the formation of amine vanadates by the action of vanadium pentoxide on the alkylamines. The simplest of 8 those that have been prepared are :

Methylamine vanadate, CH 3NH 2.HV0 3 .

Dimethylamine vanadate, (CH S ) J^H.HV0 3 . Ethylamine vanadate, (C 2H5 )NH 2.HVO 3 .

. Tetraethylamine vanadate, (C 2H5 ) 4N.V0 3 Polyvanadates or Acid Vanadates. In addition to the salts of vanadium pentoxide which have been described, there exists a large number of polyvanadates in which the molecular proportion of vanadium pentoxide to basic oxide is greater than in the case of the metavanadates* The constitution of these acid salts has been discussed on p. 62. A general method for preparing the alkali polyvanadates consists in adding acetic acid to a solution of the ortho-, pyro-, or ineta-vanadate, and concentrating. Usually several compounds of varying composition can be isolated. The polyvanadates of the heavy metals can sometimes be prepared by double decomposition of alkali vanadates with solutions of salts of these metals, but more usually complex salts containing both the heavy metal and the alkali (double vanadates) are precipitated. By precipitating a

1 Carnelly, J. Ohem. Soc., 1873, 26, 323. 2 * Carnelly, ibid., 1878, 33, 273. Canneri, kc. tit. 4 Ditte, Gompt. rend., 1887, 104, 1705. 5 Bleecker, Zoc. cit. 6 Eofmann and Kohlschiitter, Zeitsch. anorg. Chem., 1898, 16, 463. 7 Zeitsch. Kohlsckiitter and Hofmann, Annakn, 1899, 307, 314 ; Hofmann, anorff. Gfam., 1897, 15, 75. 1844. Bailey, J. Ghem. Soc., 1884, 45, 690 ; Ditte, Gompt. rend., 1887, 104, 76 VANADIUM, NIOBIUM, AND TANTALUM.

salts of solution of acid barium vanadate, 2Ba0.3V 2 5 .12H 20, with l various heavy metals, Ephraim and Beck have recently prepared a .12 or series of acid vanadates of general formula 2R"0.3V 2 5 15H aO, lose their water of or, less frequently, 3R''0.5V 2 5 ./iH 20. These crystallisation largely or completely at 220 C., with decomposition in the case of the oxidisable heavy metals. The acid vanadates are unstable towards weak acids, and are frequently decomposed even by boiling water. They are intensely coloured, usually red. The following polyvanadates are known : 0.3V Ammonium Polyvanadates. The compound (NH 4 ) 2 2 5 appears to be the most stable of the acid ammonium vanadates. It is formed by addition of a 10 per cent, solution of acetic acid to a solution of ammonium metavanadate, or by boiling an aqueous solution of any other ammonium polyvanadate. It forms thin, transparent, yellow, octagonal plates, or golden, microscopic, rhombic plates which become temporarily vermilion on being heated, and which are only slightly soluble in water. It has also been obtained combined with two, five, or six molecules of water, according to the temperature of crystallisation 2 or extent of dehydration. .4H can be looked as ammonium hexa- 2(NH 4 ) 20.3V 2O 5 2 upon It is obtained as trans- vanadate, (NH 4 ) 4V6 37 .4H 2 (see p. 63). parent, ruby-red, monoclinic crystals when a solution of ammonium oxalate, saturated at 30 C., is treated with vanadium pentoxide and 3 evaporated in a vacuum. Crystals containing 6H 2 can also be obtained. It is very soluble in water. By the action of small quantities of acetic acid of varying concentration on hot, saturated solutions of ammonium metavanadate, other 4 have been obtained : 0.2V .H or O complexes (NH 4 ) 2 2 5 20, 2H 20, 3H 2 ; 0.7V -4H 2 .10H 0.* 3(NH 4 ) 2 3 5 2 ; (NH 4 ) 20.5V 2 5 2

Barium Polyvanadates. Barium hexavanadate, 2Ba0.3V 2O 5 .12H 2O, is prepared by the addition of barium chloride to an acid solution of calcium vanadate under definite conditions of concentration and 6 acidity. Another hydrate, 2Ba0.3V 2 5 .14

Cadmium Polyvanadates. Two are known, 2CdO.3V 2O5.15H 2O and is CdO.3V 2O 5 .2H 2O. The former obtained by the action of cadmium 1 sulphate on the corresponding barium salt, and the latter by the action of cadmium nitrate on ammonium metavanadate in the presence of acetic acid. 2 Calcium Polyvanadates. Addition of calcium chloride to a solution of potassium pyrovanadate in the presence of acetic acid has furnished 3CaO.4V 2O5 .15H 2O ; CaO.2V 2O5.6H 2O ; CaO.2V 2O 5.9H 2O ; 3Ca(X .5H 3CaO.8V O .26H to 7V 2O5 .7H 2O ; 2CaO.5V 2O5 2O ; 2 5 2O, according 3 the concentration, temperature, and acidity of the mixture. Addition of calcium nitrate to excess of ammonium metavanadate solution in of nitric acid has the CaO.3V O .12H O 4 the presence given compound 2 5 2 5 which is readily soluble in water. Cerous Polyvanadate, Ce 2O 3.5V 2O5 .27H 2O, has been prepared by evaporating a solution containing ammonium metavanadate and cerous 5 sulphate. forms six-sided Cobalt Polyvanadate, 2Co0.3V 2O 5 .15H 2O, large, brown leaves which effloresce readily in air. It is obtained by the 6 action of cobalt sulphate on the corresponding barium salt. .22H is the action Copper Polyvanadate, 3CuO.5V 2O 5 2O, prepared by 7 of copper sulphate on barium hexavanadate. It forms thin, iridescent spangles." .5V .28H is obtained Didymium" Polyvanadate, "Di" 2O 3 2O5 2O, by mixing solutions of "didymium" nitrate and the acid sodium salt, 8 Na 20.2V 2 5 . 9 Gadolinium Polyvanadate, Gd 2O 3.5V 2Os.26H 2O, and Lanthanum Vanadate 10 are known. can be obtained as trans- Lead Polyvanadate, PbO.2V 2O 5 , yellow, parent needles by fusing a mixture of vanadium pentoxide, sodium 11 iodide, and lead iodide. has been as Lithium Polyvanadate, 5Li 2O.6V 2O5 .30H 2O, prepared acid to the mother- red, transparent prisms by the addition of acetic lithium metavanadate.12 If the liquor left from the crystallisation of acetic acid is added directly to a solution of lithium metavanadate or 3Li 0. orthovanadate, the compound obtained has the composition 2 has also 2Li O.3V O .15H 4V 2O5.12H 2O. The same process yielded 2 2 5 2 or 12H 13 to the or 16H 2O, Li 2O.2V 2O5 .8H 2O, 9H 2O 2O, according acidity, of the from temperature, and concentration. Evaporation mother-liquor .12H O has the the crystallisation of the compound 3Li 2O.4V 2O5 2 given 3Li O.5V O .14H O or 12H O. 14 sparingly soluble compound 2 2 6 2 2 excess of Magnesium Polyvanadates. Addition of a slight magnesium

1 Ephraim and Beck, loc. cit. 2 loc. cit. Ditte, , , T 44 and Beck, loc. cit. Manasse, loc. cit. ; also Annakn, 1887, 240, ; Ephraim 4 Ditte, loc. cit. 208. 5 Abegg, Handbuch der anorganischen Ghemie (Leipzig, 1906), 3, i, 6 cit. see also rend., 1889, 109, 148. Ephraim and Beck, loc. ; Carnot, Gompt. 7 Ephraim and Beck, loc. cit. 8 see 72. Cleve, Bull Soc. Mm., 1885, 43, 359 ; footnote, p. 9 Benedicks, Zeitsch. anorg. Ghem., 1900, 20, 393. 10 Carobbi and Bestaino, Oazzetta, 1926, 56, 59. 11 Ditte, Gompt. rend., 1883, 96, 1048. 12 938. Rammelsberg, Wied. Annalen, 1883, [ii], 20, 18 Ditte, Gompt. rend., 1887, 104, 1168. 14 Rammelsberg, loc. cit. 78 VANADIUM:, NIOBIUM, AND TAOTALUM.

sulphate to a hot solution of potassium pyrovanadate gives red crystals O. which have the composition 4Mg0.6V 2 5.19H 2 They are only at 200 C.1 slightly soluble in water and undergo dehydration When magnesium oxide is boiled with vanadium pentoxide in solution and excess of acetic acid is added, a dimorphous, soluble polyvanadate 2 .28H O is formed. having the composition 3Mg0.5V 2O 5 2 Decomposi- tion of a hot, saturated solution of ammonium metavanadate with excess of magnesium chloride and acetic acid gives another compound, .9H or 8H 3 and a heawvanadate, .19H Mg0.2V 2 5 2O 2O; 2Mg0.3V 2 5 20, has been prepared by the action of magnesium oxide on vanadium 4 oxytrichloride. Manganese Polyvanadates. The action of manganese sulphate on salt 5 acid barium vanadate has given the 2Mn0.3V 2 5.llH 2O. A .V .10H has also been 6 This can be compound 5MnO 2 2 5 3 prepared. looked upon as a vanadic manganite. Nickel Polyvanadates. NiO.2V 2O5 .3H 2 is obtained as greenish- brown crystals from the mother-liquor left after crystallisation of nickel metavanadate. The compound 3Ni0.5V 2 5 .24H 2O has been prepared 7 in two crystalline forms. Potassium Polyvanadates. By the action of acetic acid on solutions of potassium metavanadate at different temperatures and concentrations, soluble acid vanadates of the following compositions have been prepared : 8 .10H O.2V .4H ; 2K 0.4V .7H ; 3K 20.5V 2 5 2 ; K 2 2 5 2 2 2 5 2 2KA 9 excess of vanadium in 5V 2 5.12H 20. By dissolving pentoxide 10 potassium carbonate solution and then adding acetic acid, Ditte also 0,2V .3H or or isolated the compounds K 2 2 5 2 8H 2 10H 20, and or 5H O or 6H 0. l:l The K 20.3V a 5 ,H 2 2 2 anhydrous compound is insoluble in water. K 2O.3V 2 5 almost Potassium Hexavanadate, 2K 2O.3V 2O5 .6H 2O or K 4V6 17 .6H 20, is obtained as orange plates by concentration of the mother-liquor from 12 salt . The K V O 7H has also been the K 20.2V 2 5 heptahydrate, 4 6 ir 20, 13 prepared. When vanadium pentoxide is fused with potassium chloride and the product extracted with water, the following are obtained : 0.2V .4H or the 22K 20.24V 2 5.7H 2 ; K 2 2 5 2 6H 2 ; and insoluble residue is found to contain the compound 2K 2O.SV2O5.3H 2O. Sub- stitution of potassium fluoride for the chloride in this reaction gives a vanadate which contains the highest proportion of vanadium pentoxide 14 in the series, namely, 2K 2O.9V 2 5 . Samarium Polyvanadates. Addition of a neutral samarium salt to a solution of a metavanadate throws down a yellow, amorphous precipitate which consists mainly of samarium orthovanadate. Two acid

1 Manasse, Annalen, 1887, 240, 49. 2 loc. cit and J. Manasse, j Sugiura Baker, Chem. Soc*, 1879, 35, 715. 3 von Hauer, loc, cit. * Cuttica, Tarchi, and Alinan, Gazzetta, 1923, 53, 189. 5 Ephraim and Beck, loc. cit. 6 barker and Dhar, Zeitsch. anorg. Chern., 1922, 121, 135. 7 Ephraim and Beck, loc. c,\t. 8 Badau, Annakn, 1889, 251, 114 ; Norblad, Dissertation (Upsala University, 1873). 9 Manasse, Annalen, 1887, 240, 23. 1 Ditte, Gowpt. rend., 1887, 104, 902. 11 Norblad, loc. cit. 2 1 Ditte, loc. cit. 13 Friedheim, Ber., 1890, 23, 1526. 14 Ephraim, Zeitsch. anorg. Chem*, 1903, 35, 66. COMPOUNDS OF VANADIUM. 79 samarium Sm O .5V O .28H vanadates, namely, 2 3 2 5 2O and Sm 2O 3.5V 2O 5 . 1 24H 2O, have been isolated from the mother-liquors. Sodium Sodium Polyvanadates. liexavanadate, Na 4V6 O17 or 2Na 2O. is a 3V 2O5 , prepared by acidifying solution of sodium metavanadate. It has been obtained 2 3 combined with 18H 2O, 16H 2O, 10H 2O, and 4 9H 2O. The anhydrous salt is almost insoluble in water. From electrical conductivity data under different conditions, Dullberg 5 was able to show the existence in solution of liexavanadic acid, H 4V6 O17 , which rise also to the salts : gives following acid Na 3HV6 O17 .13H 2O and Na 2H 2V6O17 .2H 2O. It is by no means certain, however, that these salts are derivatives acid of hexavanadic acid ; they can be alternatively as formulated Na 2O.2V 2O5 .9H 2O and Na 2O.3V 2O 6 .8H 2O respectively. The former has been prepared by the action of acetic acid on sodium 6 its metavanadate ; pentahydrate, Na 2O.2V 2O5.5H 2O, and decahydrate, 7 Na 2O.2V 2O5 .10H 2O, are also known, both of which undergo dehydration at about 200 C. the residue melts at 581 C.8 ; The compound Na 2O.3V 2O5.8H 2O has been made by the action of caustic soda on 9 excess of vanadium pentoxide. Its pentahydrafe, Na 2O.3V 2O5 .5H 20, 10 and nonahydrate, Na 2O.3V 2O5,9H 2O, are also known. All these sodium salts are red, easily crystallisable substances. Slight variations in the method of preparation have yielded several further of : polyvanadates sodium 5Na 2O.8V 2 5 .39H 2O ; 4Na 2O. " 12 7V 2O6 .33H 2O ; 3Na 2O.5V 2O 5.22H 2O or 23H 2 ; 4Na 2O.7V 2O5 . 13 14 35H 2O ; 3Na 2O.7V 2O5 .33H 2O ; 2Na 2O.5V 2O5.28H 2O ; 4Na 2O. 15 16 10V 2 5 .7H 2 ; 2Na 20.8V 2 5.15H 2O or 17H 2O. Strontium Polyvanadates. By the action of strontium chloride on potassium metavanadate solution in the presence of acetic acid, the 17 following compounds have been isolated : 3SrO.4V 2 5.14H 2O ; 18 4SrO.7V 2O 5 .30H 2O ; SrO.2V 2O 5.9H 2O. The last-mentioned salt has also been obtained from the mother-liquors left after the separation 19 of strontium pervanadate. Using the compound K 2O.2V 2O 5 in place 20 of the metavanadate yields the more acid salt, 2SrO.6V 2O 5 .27H 2O, A polyvanadate containing four molecules of vanadium pentoxide, viz. 21 SrO.4V 2O5 .llH 2O, has also been prepared. Thallium Polyvanadate. By treating ammonium metavanadate with 1 Cleve, Bull. Soc. chim., 1885, 43, 162. 2 Rammelsberg, Wied. Annalen, 1883, pi], 20, 934 ; Ditte, Compb. rend., 1887, 104, 1061. 3 Norblad, loc. cit. 4 Dullberg, Zeitsch. physical. Chem., 1903, 45, 175. 5 loc. cit. 779. Dullberg, ; but compare Canned, Ctazzetta, 1923, 56, ii, 6 von Hauer, loc. cit. 7 Ditte, loc. cit. 8 Carnelly, /. Chem. Soc., 1878, 33, 273. 9 Ditte, loc. cit. * 393. Norblad, loc. cit. ; Prandtl and Lustig, Zeitsch. anorg. Chem., 1907, 53, 11 Friedaeim and Michaelis, ibid., 1893, 5, 441. 12 Prandtl and Lustig, loc. cit. ; von Bex, Dissertation (Bern, 1901). 13 Schmitz-Dumont, ibid. (Berlin, 1891). 14 Botnenbach, ibid. 15 Bammelsberg. loc. cit. 16 Baragiola, Dissertation (Bern, 1902). 17 Manasse, Annalen, 1887, 240, 43. 18 von Hauer, loc. cit. 19 Scheuer, Zeitscli. anorg. Ghem., 1898, 16, 284. 20 cit. 156. Manasse, loc. ; von Hauer, J. prakt. Ghem., 1859, 76, 1 2 Manasse, loc. cit. 80 VANADIUM, NIOBIUM, AND TANTALUM. has been thallium sulphate, a red acid salt of composition 6T1 2O.7V 2O5 1 2 obtained. It melts at 408 C. A kexavanadate, T1 2 0.3V 2O5 , has 3 also been prepared. 4 is known. Thorium Polyvanadate, Th0 2.6V 2 5 .SH 20, .3H and .15V O Ytterbium Polyvanadates, 3Yb 2O 3 .5V 2O5 2 Yb 2 3 2 5> are known. 5 is obtained as well-defined Zinc Polyvanadate, 2Zn0.3V 2 5 .15H 20, vanadate. 6 crystals by the action of zinc sulphate on acid barium Double Vanadates. Vanadium pentoxide exhibits a strong tendency to combine with two or more bases to produce well-crystallised double salts. Their general method of preparation consists in adding a solution of a salt of the required metal to a solution of an alkali of the after of vanadate ; concentration mother-liquor precipitation is to contain the simple vanadate gives rise to a vanadate which found both the alkali base and the metallic base. Frequently from the same two bases and vanadic acid a series of such double vanadates can be of basic and vanadic prepared in which the relative proportions oxides acid vary. The decomposition of ammonium metavanadate with acetic acid in the presence of sodium silicate yields glistening red O.Na 0.5V .15H 0. The molec- crystals of the composition 2(NH 4 ) 2 2 2 5 2 a ular proportions in this compound are simple, but large number of double vanadates have been prepared in which the molecular pro- the action of two molecular portions are by no means simple. Thus, by chloride on a concentrated solution proportions of ammonium boiling, of one molecular proportion of sodium metavanadate, yellowish-red O are obtained. 7 prisms of composition 4[(NH 4 )tf.Na|f]0.7V2 5 .17|H 2 It is a matter of some doubt whether these complex double salts should be looked upon as definite compounds, each having independent existence, or whether each series is not more correctly regarded as composed of isomorphous mixtures of variable composition. The problem is similar to that of the constitution of the salts of the heteropoly-acids, in which vanadium pentoxide is combined with other acid anhydrides. Other double vanadates which have been prepared are : 0.4V .3H or 4NH V0 .4KV0 .3H 2(NH 4 ) A2K 2 2 5 2 4 3 3 2 ; 8 0.20V ,52H ; 3(NH 4 ) 2O.9K 2 2 5 2 9 O.K 0.5V .9H ; 2(NH 4 ) 2 2 2 5 2 or 4Na .K V O .35H 8Na 20.2K 20.18V 2 5 .35H 2 4V6 17 4 6 17 2 ; or .2K O .30H 6Na a0.4K 20*15V 2 5 .30H 2 3Na4V6 17 4V6 I7 2 ;*

4K 2O.Ca0.10V2 5 .22H aO ;

K O.3SrO.7V O .20H 2 and 30H 2 ; 2 2 5 " 2K 20.2Sr0.7V 2 6.18H 2 ;

K 2O.2ZnO.5V 2O5 .16H 2O ;

1 Carnelly, J. Ohem. Soc., 1873, 26, 323. 2 Carnelly, ibid., 1878, 33, 273. 3 Cuttica, Tarchi, and Alinari, Gazzetta, 1923, 53, 189. 4 Cleve, Jahresber., 1874, 261. 5 Cleve, Zeitsch. anorff. Chem., 1902, 32, 129. 6 Ephraim and Beck, loc. cit. 7 Baragiola, Dissertation (Bern, 1902). 8 Baragiola, loc. cit. 8 Ditte, Compt. rend., 1887, 104, 1844. 10 Friedneim and Michaelis, Zeitsch. anorg. Chem., 1894 5, 441. 11 Manasse, Annalen, 1887, 240, 46. COMPOUNDS OF VANADIUM. 81 0.7V 2(4K 2 2 5.22jH 20)+3(4Zn0.7V 2 5-22iH 20)or

16K 2O.24ZnO.70V 2O 5 .225H 2O ;

K 2O.CdO.3V 2O5 .9H 2O ;

3K 2O.3CdO.10V 2O 5 .27H 2O ; K O.7MnO.8V O .25H O or 2KVO 2 2 5 2 3.7Mn(VO 3 ) 2 .25H 2O ; K 0.1lMn0.12V .48H or 2KVO 2 2 5 2 3.llIVln(VO 3 ) 2.48H 2O ;

K 2O.2MnO.5V 2O 5 .15H 2O and 16H 2 ; 0.5Ni0.6V .27H or K 2 2 5 2 2KVO 3.5Ni(VO 3 ) 2.27H 2O ;

K 2O.3NiO.5V 2O 5 .17H 2O ;

2K 2O.4NiO.10V 2O 5.33H 2O ;

2K 2O.NiO.7V 2O 5 .23H 2O ;

K 2O.2CoO.5V 2O5 .16H 2O and 16}H 8O ;

K 2O.3CoO.7V 2O5 .2lH 2O ; * 2K 2O.4CuO.18V 2O5 .69H 2O ; 2 BaO.CdO.3V 2O 5 .9H 2O ; 3 3CdO.3ZnO.10V 2O5 .27H 2O. Many of the naturally occurring ores of vanadium have also the composition of double vanadates (see table on p. 11).

HETEROPOLY-ACIDS CONTAINING VANADIUM.

General. Vanadium possesses the property of entering into the composition of a large number of compounds which contain it as a constituent of a complex anion. Light has been thrown on the nature and constitution of these compounds by investigations into other heteropoly-acids, which are now found to be most satisfactorily formulated by the application of a modification of Werner's co-ordination 4 theory first suggested by Miolati, and extended by Rosenheim and his 5 co-workers. According to this theory the heteropoly-acids are proof acids the _ n duced by hydration having general formula H8 n [R O 4], where n represents the valency of the metal R. Thus, for example, in the cases of silicon and phosphorus the following schemes obtain :

H 4[Si0 4]+2H 20=H8 [Si0 6 ]. H 3rP0 4]+2H 20=H7 [P06 ]. Or, generally, _ H8-W[R"0 4] +2H 2 =H12 n[R0 6 ]. It will be observed that the basicity of each of these acids is determined by deducting the valency of the central atom in the complex anion from 12. The greatest number of oxygen atoms or other divalent radicals

that can be co-ordinated with the central atom is 6 ; and this number usually holds good in a true compound, but not always. For example. the co-ordination number of the acid H P^ n 3 ' 2 is only 4. L ^2 J Substitution of the co-ordinated oxygen atoms by divalent radicals yields the various heteropoly-acids. The vanado-phosphates, for instance, are derivatives of the hypothetical phosphato-acid H 7 [PO6],

1 Radau, Chem. Zentr., 1888, 1378; Annakn, 1889, 251, 114. Other double salts of potassium and manganese have been prepared. 2 Weinland and Feige, Ber. 9 1903, 36, 260. * Radau, foe. cit. * Miolati, J. prakt. Chem., 1908, 77, 439. 5 for a See Rosenheim and Janicke (Zeitsch. anorg. Ghem. 9 1917, 100, 304) compre- hensive review of the subject.

* VOL. YI. : III. 6 82 VANADIUM, OTOBIUM, AND TANTALIM. in which one or more of the oxygen atoms have been replaced by radicals. The to which the formula divalent (V 8 6 )" compound .12V .26H has been ascribed from its analytical 7(NH 4 ) 2O.P 2 5 2O 5 2 O O. If some of the oxygen data thus becomes (NH4 ) 7 [P(V 2 6 ) 6 ].13H 2 O radicals and atoms in the complex anion are replaced by (V a e )" are pro- others by (Mo 2 7 )" radicals, Jthe molybdwanado-pkosphates and duced, e.g . (NHp lH/) and radicals Similarly, substitution by both (V 2O6 )" (W 2 7 )" gives rise to the tungstovanado-phosphates. The heteropolyvanado-arsmates It has are derived from the hypothetical arseno-acid H7 [As06 ], recently been shown that the arsenic in these compounds may undergo partial substitution by phosphorus to give rise to series of mixed crystals which are called hetewpolyvanado-arsenophosphates or heteropolyvanado- derived from the phosphoarsenates.'*- The heteropotyvanado-silicates are

acid H8 [SiO6 ]. as and The existence of such associated radicals (V aO6 )", (Mo 2 7 ) , has been assumed in order to the frequent (W 2O 7 )" explain very or radicals in the occurrence of twelve (jv a 5 ), MoO s , W0 3 complex anions of compounds the co-ordination number of which is six. A greater number than twelve has never been observed. As a rule only four of radicals the oxygen atoms can undergo substitution by these associated ; thereafter an isomerie change appears to take place, thus :

3 Compounds are said to be saturated when all the oxygen atoms of the metallic acid anions unsaturated com- parent anion are replaced by ; pounds contain some replaceable oxygen. The decision as to which element forms the central atom of the complex anion depends on the fact that in any particular series of compounds the atomic proportions of the metals in the place of the co-ordinated oxygen will show very considerable variation when calculated for one atomic weight of the element which constitutes the central atom. Among the molybdo-vanadophosphates many are known which contain varying amounts of molybdenum and vanadium in combination with one gram-atom of phosphorus. Hydrogen also provides the nuclear atoms in some series, which are best viewed as O 2 derivatives of a hypothetical co-ordinated hexa-aquo-acid, H10 [H a 6]. The vanado-tungstates (or tungsto-vanadates) are, for instance, represented this the formula H10 of type by general \^S\i/c^\V T Compounds L V *2^7/6-aU contain water of constitution. It should be noted that vanadium does not as a rule form the central atom of the complex anion in the heteropoly-compounds. The oxalo- r (c 2o 4/2) T" vanadates, however, most probably contain the anion Vy* , which is obtained by substituting two oxygen atoms in the anion of orthovanadic acid, i.e. the [VO 4]'" ion by two (C 2O 4 )" groups. Sub- stitution by (MoO 4 )" groups gives rise to the ocealo-molybdovanadates of the general formula :

1 Canneri, GazzeUa, 1926, 56, 871. 2 Ann. Chim. 118 217 Copaux, Phys., 1906, 7, , 1909, 17, ; 1912, 26, 22, COMPOUNDS OF VAN4JHTM. 83

or I. ) J Uranyl-vanadates, which are probably derived from uranyl-vanadic acid, 3 H VQ L have also recently been prepared. The constitution of many of the heteropoly-acids and of their salts may thus be explained, although direct proof of the correctness of some of these formulae is lacking. Some of the heteropoly-compounds of vanadium are, however, very complex, and cannot be represented by application of the foregoing scheme. To take an extreme instance, * Rogers prepared a black, crystalline compound to which he ascribed the following formula :

4 ) 20.12P 2O 6 .2As 2O5 - 6^ 2O 5 .6V O .19lW0 .522H O. 99(NH 2 3 3 2 The composition of these substances is necessarily a matter of doubt, because of the difficulty of conducting exact analyses of compounds of molecular a small very high weight ; difference in the analytical data makes a large difference in the number of molecules. Further, the heteropoly-compounds are characterised by the ease with which they even the of undergo hydrolysis ; process recrystallisation frequently produces a different complex. In many cases the salts of any one series which have been analysed are not true compounds, but consist of isomorphous mixtures of simple chemical compounds ; the composition of the mixtures varies with the composition, temperature, and acidity of the media from which they separate. This has been shown to apply to the molybdo-vanadates 3 (or vanado-molybdates)? molybdo-vanadophosphates, vanado-selenous 5 acid and the vanado-selenites,* as well as to telluric acid and its salts. 6 Recently, Canneri has succeeded in preparing a large number of mixed crystals of salts, and in some cases of the free acids, which to different series for of belong ; example, mixed crystals tungsto- vanadophosphates and tungsto-vanadoarsenates, of tungsto-vanadophosphates and molybdo-vanadophosphates, of tungsto-vanadoarsenates and molybdo-vanadoarsenates ; and, finally, mixed crystals of members of all the four series mentioned have been obtained. The mixed crystals have the same crystalline habit as that of their components, and complete isomorphism exists between any two series which mix in all proportions to form continuous series of mixed crystals containing four or five different oxygenated acid radicals. The crystallographic data of a number of ammonium tungsto-vanadoarsenates have also recently been independently determined, and it has been shown that the crystal form is unaffected when the arsenic is substituted by phosphorus, or the substituted 7 when (W 2 7 )" radicals are partially by (Mo 2O 7 )". It appears probable that adsorption of uncombined radicals by molecules which contain a large number of these radicals, but which

1 Rogers, J. Amer. Ohem. Soc., 1903, 25, 298. 2 See infra. 3 /. Cain andHostetter, J. Amer, Chem. Soc., 1921, 43, 2552 ; Wherry, Franklin (n&t,, 1910, 169, 486. 4 Rosenheim and Kranse, Zeitsch. anorg. Chem., 1921, 118, 177. 5 Rosenneim and Jander, Kolloid. Zeitech., 1918, 22, 23. 6 Canneri, Gazzetta, 1926, 56, 871. 7 Rodolico, AUi 22. Accad. lincei, 1926, [vi], 4, 471. 84 . VANADIUM, OTOBIUM, AND TANTALUM. true chemical indi- are not saturated, takes place, and that the only viduals are the maximum co-ordinated compounds. It has been shown that vanadium pentoxide forms a colloidal solution, in which state it is 1 readily adsorbed. Vanado-phosphates. Alkali vanado-phosphates are numerous, and contain from one to twelve molecules of vanadium pentoxide in combination with one molecule of phosphorus pentoxide. Those with and high vanadium pentoxide content are red, crystalline substances, low vanadium are called ; those with pentoxide pwpureo-compounds 2 content form yellow crystals, and are styled teo-compounds. Series. These are prepared by dissolving vanadium pent- Purpureo 8 oxide in solutions of alkali phosphates. The following are known :

O.P .12V .26H or 4 ) 7 2 6 ) 8]-13H 2 ; 7(NH 4 ) 2 2 5 2 5 2 (NH [P(V .12V .44H or H 2O ; 5(NHj) 2O.P..2 5 2 5 2 (NH 4 ) 5 2[P(V 2 6 ) 6].2lH .26H or ; 7K 2O.P S 5 .12V 2 5 2 K7 [P(V 2 6 ) 6 ].13H 2 .26H or K H O 2 ; 5K 20,PA.10V 2 5 2 5 2[P(V 2 fi ) 50].12H to the last together with crcsium and rubidium compounds analogous member of the above. Luteo Series. These are prepared by adding excess of phosphoric 4 acid to solutions of alkali vanadates. The following are known :

or 3 O.P -V 5 -1 or 3H 2 (NH 4 20; (NH 4 ) 2 2 5 2 )H[p^ ].2H

( s)2 O.P .2V .5 or 7H O or 4 .5 or 7H 2 ; (NH 4 ) 2 2 5 2 5 2 (NH )p Q

or 2K 2O.P 2 5 .V 2 5

-6 or O.2P .2V .5H O K 2O.P 2 5 .2V 2 5 7H 2O ; 3K 2 2 5 2 5 2 ; 3K 2O^PA-6V 2 A free acid having the composition

2HV0 3.2HP0 3 .9H 2 or has been obtained in golden-yellow leaves by the action of vanadium pentoxide on syrupy phosphoric acid. Molybdo-vanadophosphates. These are derived from the hypo- in which the co-ordinated atoms have thetical acid H7 [P0 6 ], oxygen or radicals and been partly replaced by one more (V 2 6 )" partly by (Mo A)" radicals. The relative proportions of these radicals present may undergo variation with constant phosphorus content, so that a phosphorus atom must form the central atom of the complex anion. As the phosphorus content also varies, it is obvious that a very large number of acids becomes possible. These are usually heptabasic, hexabasic, or pentabasic, and the ammonium, potassium, and barium salts, as well as the barium-ammonium and barium-potassium double salts of many of them are known. Their general method of preparation consists in acidifying solutions containing molybdates, vanadates, and

1 Gessner, Kolloidchem. Beihefte, 1924, 19, 213. 2 Friedheim and Szamatolski, Ber., 1890, 23, 1530, 2COO ; Bosenlieim and Pieck, Zeitsch. anorg. Chem., 1916, 98, 225; Rosenheim and Yang, ibid., 1923, 129, 181. 3 See also Friedheim and Miohaelis, ibid., 1894, 5, 446. 4 See also Friedheim and Michaelis, loc. cit. ; Oibbs, Amer. Chem. J., 1885, 7, 209 ; Ditte, Compt. rend., 1886, 102, 1019. COMPOUNDS OF VANADIUM. 85 phosphates. They form isomorphous, beautiful orange-red to brown, octahedral crystals, which undergo hydrolysis very readily, so that it is frequently impossible to state the experimental directions whereby a salt of definite composition can be produced. A few examples only of these compounds are here set out. For the exhaustive list of those hitherto prepared and analysed, the references cited should be consulted. 1 O.P .6V O .12MoO .4lH 6(NH 4 ) 2 2 5 2 5 3 2O or (NH4 ) 6 20H 2 ; H[P|VA)^ J. 5K 2O.P 2O5 .2V 2O 5 .20MoO 3.53H 2O or .8V 6K 20.(NH4 ) 2O.P 2O 5 2O 5 .18Mo0 3.43H 2O

or I .39H 3BaO.3(NH 4 ) 2O.P 2O 5 .4V 2O 5.14MoO 3 2O. Many of the compounds in this class cannot be represented by co- ordinative formulas. Tungsto-vanadophosphates. These are analogous to the molybdo-vanadophosphates, only differing from them in that the radical in place of the (Mo 2O 7 )" the complex anion is now taken by radical. the (W 2O 7 )" Thus, an ammonium tungsto-vanadophosphate

from its has the formula . which, analytical data, 6(NH 4 ) 2O.P 2O5 4V .16W0 .21H is written HTl> a ) and a 2 5 3 20, (NH4 )a !w ?f ?l-20H L ("pJrfj&j A barium salt which has the composition 18BaO.3PoOs.2V 2O s .60WOo. y r ( A) -] 144H becomes P .14lH O. of 2O 9BaH 3 3 (W 2O7 ) 15 2 A number L 2 J ammonium, potassium and barium salts have been prepared. The 2 ammonium salts in some cases approach to the following series : H (N 4 )6H[

By the action of dilute sulphuric acid on the barium salts several free 3 tungsto-vanadophosphoric acids have been isolated. Vanado-arsenates. The salts of this class which have hitherto been prepared are all of low vanadium pentoxide content compared with the amount of arsenic pentoxide present. They correspond, therefore, to the luteo vanado-phosphates. Three series are known. The first has the general formula R"O.V2O5.As2O5 .#H2O or R" where R" may be Mg, Zn, Cu, or Co. They form yellow crystals which are obtained by the addition of arsenic pentoxide to solutions 1 Gmelin-Kraut, Handbuch der anorganischen Chemie (Heidelberg), 1908, 3, ii, 205-213, 1095-1104. 2 Gmelin-Kraut, ibid., pp. 187-188; Bodolico, Atti E. Accad. Lincei, 1926. [vi],

4, 471 ; Caxmeri, Gazzetta, 1926, 56, 642. 3 Canneri, ibid., 1926, 56, 871. 86 VANADIUM, NIOBIUM, AND TANTALUM. of the carbonate of the of the respective vanadates, or by the action Of the second metal on the free vanado-phosphoric acid (see below). are viz. series only the ammonium and potassium salts known, 8) or 0, and K 2O. 20.2V 2 5.As 2 5 .5H 2 (NH 4 8 (NH 4 ) )[AsjJ ].5H 3)a also rise to 2V .As .5H or 20. These give 2 5 2 5 2 K[As^ ].5H has the formula 2R* O.2V 2C>5. yellow crystals. The third series general 2 ere R' be Ca, Sr, Zn, Mn, Co, NL 3As 2O 5.0HA wk 2 may (NH 4 ) 25 Mg, of They result from the action of arsenic pentoxide on solutions are and vanadates, and form red crystals which, however, unstable, of the first series. 1 are readily converted by water into compounds The free acid of the first series has been obtained in yellow plates with a solution of arsenic acid. Its by boiling vanadium pentoxide 3 0. is V O .As O .10H or H 2 2 Recrystallisa- composition 2 5 2 5 2 [As^ ].4H a tion of this hydrate from concentrated nitric acid gives dihydrate, 3 V O .As O .2H 0, which can be looked upon as the acid 2 5 2 s 2 H^As^ J. .As is obtained the to The anhydride, V 2 5 2 5 , by heating dihydrate 440 C. Molybdo-vanadoarsenates. A number of compounds have been described which are analogous to the molybdo-vanadophosphates described above, and which contain arsenic for the nuclear atom of the to the complex anion. In many cases these compounds approximate x and formula B/ lnH 20, where %+y=6 ; y general 6HrAs|^^ shown are not necessarily whole numbers, because of the tendency by of these heteropoly-acid salts to form isomorphous mixtures simple been compounds. Ammonium, barium and thallium salts have 2 prepared. Tungsto-vanadoarsenates. These are comparable to the molybdo-vanadoarsenates, and can be represented generally,

where #+j/=6, although tc and y are not necessarily whole numbers. Canneri found a definite relation to exist between the composition of the salt obtained and that of the solution from which it separates, from which it is inferred that a condition of equilibrium exists between various this for fixed con- salts in solution or between their ions ; equilibrium centrations is remarkably sensitive to changes in temperature or acidity. Among other salts three series of ammonium salts have been obtained which approach to the following formulae :

(NH4 )aHAs .nH.0 ; (NH4

1 Zeitsch. Gibbs, Amer. Chem. /., 1886, 7, 209 j Friedheim, Ber., 1890, 23, 1530, 2600 ; anorg. Chem., 1892, 2, 319; Schmitz-Dumont, Dissertation (Berlin, 1891); Fernandez,

er., 1884, 17, 1632 ; Ditto, Oompt. rend., 1886, 102, 757, 1019. 2 Canneri, Cfazzetta, 1923, 53, 773 ; 1926, 56, 871. COMPOUNDS OF VANADIUM. 87 Barium and thallium salts have also been described, and some of the free acids have been isolated. 1 Tungsto-vanadoarsenophosphates. Several ammonium salts have been which are prepared, most probably isomorphous mixtures of 2 arsenates and phosphates.

Vanado-tungstomolybdoarsenophosphates have been described ; these are also isomorphous mixtures of simple compounds. 3 Molybdo-vanadates or Vanado-molybdates. These are obtained in red rich in crystals, vanadium, or yellow powders, poor in vanadium, in by dissolving molybdic anhydride, Mo0 3 , solutions of alkali metavanadates, or by acidifying mixed solutions of molybdates and vanadates. A number of salts have been large prepared, which vary considerably in their data. analytical Repetition of the same process of preparation often gives crystals of different composition, so that a definite compound cannot easily be prepared. The salts are usually unstable, and undergo change even on being recrystallised. The same solution may give rise to different salts according to the temperature. Thus, one of a solution divided into two of 0. portion parts gave crystals 2(NH 4 ) 2 2V .5Mo0 .10H at 10 at 2 5 3 2 C., while 30 C. the other portion gave of 0.3V .5Mo0 .10H 0. crystals 4(NH4 ) 2 2 5 3 2 From a study of the V0 Mo0 NH ; KV0 Mo0 it , ; NaV0 systems 4 39 3 3 3 3 , Mo0 3 , has been concluded that solutions of alkali molybdo-vanadates of fixed concentration and at a definite temperature contain definite compounds which are in equilibrium with isomorphous mixtures of polyvanadates and polymolybdates. The yellow powders poor in vanadium are presumed to be definite compounds because they resist fractionation, while the 4 red crystals consist of isomorphous mixtures. Comparison with other heteropoly-acids strongly suggests that the molybdo-vanadates do not contain either vanadium or molybdenum as the central atom of the complex anion. They are therefore written as derivatives of the in hypothetical hexa-aquo-acid, H10 [H 2 6 ], which the oxygen atoms in the anion have undergone complete or partial radicals or radicals. replacement by (V 2 6 )" and by (Mo 2 7 )" (Mo0 4 )" The following examples are typical of series of alkali (and in some cases * barium) salts that have been prepared :

20 3 0.2V .4Mo0 .a?H or 9HrH 2 , l.nH aO; 3(NH 4 ) 2 2 5 3 2 (NH4 ) (J!r ^ r (v,o.). i 0.2V .12Mo0 .0H or H H O 5(NH 4 ) 2 2 5 3 2 5(NH4 ) 2 10 4 (Mo 2 7 ) 6 L(aj-7)Ha L 4 J

1 loc. cit. J. Amer. Ofam. 298 ; Canneri, Gazzetta, Gibbs, ; Rogers, Soc., 1903, 25, 871 loc. cit. 1923, 53, 773, 779 ; 1925, 55, 883 ; 1926, 56, ; Rodolico, 2 cit. kc. cit. foe. cit. Rogers, kc. ; Rodolico, ; Canneri, 3 312 loc. cit. loc. cit. Rogers, J. Amer. Ohem. Soc., 1903, 25, ; Rodolico, ; Canneri, 4 Canneri, Qa&etta, 1923, 53, 779 ; 1925, 55 390. 5 Gmelin-Kraut, Handbuch der anorganischen Ghemie (Heidelberg), 1908, 3, ii, 191-204, /oc.c^. 1092-1094; Abegg and Auerbach,i6^. (Leipzig), 1921, 4, i (second half), 1052; Canneri, 88 VANADIUM, NIOBIUM, AND TANTALUM. others for which co- The following are also known, as well as many

ordinative formulae cannot be written :

0.3V .5Mo0 .10H or 4(NH 4 ) 2 2 5 3 2 L \ 26 / 3

) 4K 0.8V or K HlH i a l 5.4MbO,.7H,0 8 jWV I \ 26/3 Tungsto-vanadates or Vanado-tungstates. Three well-defined have been series of complex salts containing tungsten and vanadium that obtained, which are comparable to the molybdo-vanadates in they as the nuclear probably do not contain either vanadium or tungsten saturation of atom of the complex anion. They are obtained (1) by - the a solution of a paratungstate with vanadium pentoxide, (2) by action of acetic acid on solutions of mixed alkali tungstates and to a metavanadate. vanadates, or (3) by the addition of a paratungstate water of con- The salts are characteristically coloured and all contain stitution. The dichromate-coloured series has the composition 5R' 20.

or where R' 3V 2 5 .6W0 3.0H 2 R-gHHa^.^-^^p, may be or Na. The salts have the NH 4 , K, yellowish-red composition salts of the alkali alkaline earth 2R* 2O.V 2 5.4WO S.#H 20, and metals, series metals, and silver have been prepared. The salts of both these salts of the third have properties similar to the paratungstates. The 5R' 0.3V J4!W0 . series are deep red in colour, have the composition 2 2 5 3

tfH 20, and resemble the metatungstates. The ammonium, potassium, 1 ccesium, and barium salts have been prepared. Uranyl-vanadates. -By adding uranic anhydride to fused potassium or sodium metavanadate, microscopic, rectangular, fluor- are escent plates of the compositions K(U0 2 )V0 4 and Na(U0 2 )V0 4 obtained. These are probably derivatives of a relatively stable uranyl- 3 vanadic L and are to the acid, Hj VQ analogous uranyl-phosphates 2 which occur naturally. Molybdo-vanadosilicates. The simple vanado-silicates do not appear to be stable, but a number of molybdo-vanadosilicates have been obtained either by acidifying solutions which contain molybdates, vanadates and silicates, or by addition of vanadium pentoxide to the molybdo-silicates. They form heavy, yellowish-red to brownish-red crystals, which are very easily hydrolysed and are, therefore, not always reproducible. The following isomorphous series has been recognised :

v (VA)V 2^6/22 3R O.Si0 .V .9Mo0 .0H or Si 2 2 2 5 3 2 2 (Mo 2 7 ) 9 .(20-2)H,0;

^ r~ fv r

O.Si0 .V .o?H .o;H 3R 2 2 2 5.llMo0 3 2 ; 3R 2O.Si0 2.V 2 5.15Mo0 3 aO.

(R=NH 4 orK; R 2 =Ba.)

1 loc. Gmelin-Krant, cit., pp. 179-188 ; Prandtl and Humbert, Zeitsch. anorg. Chem., Prandtl and 1911, 73, 223 ; Hecht, ft&, 1915, 92, 198 ; Sweeney, J. Amer. Chem. Soc. t 1916, 38, 2377 ; Bosenheim and Pieck, Zeitsch. anorg. Chem., 1916, 98, 223. 2 Canneri and Pestelli, Gazzetta, 1924, 54, 641 ; compare Carnotite (see p. 10). COMPOUNDS OF VANADIUM. 89

Potassium-ammonium double salts have also been prepared. The first two series can be looked upon as derivatives of the co-ordinated silico- lv acid, H8 [Si O6 ] ; the last two series cannot be formulated in accordance 1 with Werner's theory. Tungsto-vanadosilicates. These complexes are analogous to the molybdo-vanadosilicates, and are prepared as yellowish-red to red the crystals by action of vanadates on the silico-tungstates, or by the action of hydrofiuosilicic acid on the tungsto-vanadates. The following series has been recognised :

3R 2O.SiO 2 .V 2O 5 .9W0 3.#H 2O ; O.SiO .V 3R 2 2 2O 5 .10WO 3.a?H 2O ;

7R 2O.2SiO 2.3V 2O 5 .18WO 3.0H 2O.

(R=NH 4 orK; R 2 =Ba.) Double salts have also been isolated. 2 Several of the natural ores of vanadium consist of complex silicates, e.g. ardennite and roscoelite. Vanado-selenites. Three series of alkali vanado-selenites are known, the members of which are all prepared by treating solutions of alkali vanadates with selenous acid in varying proportions, or by acidifying mixed solutions of vanadates and selenites with acetic acid. The members of the first series are yellow, and have the composition R* 2 O.V 2O5.2SeO 2 .#H 20. The second series of salts is orange, and includes types of varying composition : R' 2O.3V 2O5.4SeO 2.#H 2O, R* 2O.6V 2O5 .8SeO 2 .o?H 2O., as well as other types which contain greater proportions of SeO 2 . The salts belonging to the third series are red, O. and have the composition 4R* 2 O.6V 2O5 .5SeO 2 .#H 2 All these vanado- selenites lose selenous oxide on being boiled with water. It has been shown that when the same amount of ammonium metavanadate is boiled with selenous acid solutions of gradually increasing concentration, the yellow crystals obtained contain gradually increasing proportions of selenous oxide. It appears, therefore, that the large number of vanado-selenites known, and their complexity, are due to their semi- colloidal character, in consequence of which they adsorb constituents 3 from solutions. As might be expected, only a few of them can be represented by co-ordinative formulae. According to Prandtl, the second 4 and third series above are derivatives of hexavanadic acid, H 4V 6O17 . By the action of vanadium pentoxide on selenous acid solutions, or by reduction of a solution of vanadium pentoxide in selenic acid, red crystals of a free vanado-selenous acid have been obtained, the com- O.<2 It contains four mole- position of which is 3V 2O6 .4SeO 2.4H 2 aq. cules of water of constitution, and, according to Prandtl, should be formulated H 4V6 O17 .4H 2SeO 3 .(a?--2) aq. Vanado-tellurites. These compounds are of interest in that they are the only heteropoly-acid compounds containing tetravalent tellurium. They are prepared by mixing hydrochloric acid solutions of tellurium dioxide with solutions of vanadium pentoxide in caustic soda and then neutralising with more hydrochloric acid, or by mixing strongly alkaline 1 Friedheim and Castendyck, Ber., 1900, 33, 1612. 2 Friedheim and Henderson, ibid., 1902, 35, 3243. 8 Rosenheim and Krause, Zeitsck. anorg. Chem., 1921, 118, 177. 4 Zeitsch. 393 ; Prandtl and Lustig, JBer., 1905, 38, 1307 ; arwrg. Chem., 1907, 53, 779 Zeitech. 1911, 73, 223 ; compare Canneri, Gazzetto, 1923, S3, ; Rosenheim, anorg. Chem., 1915, 93, 273. 90 VANADIUM, NIOBIUM, AND TANTALUM. solutions of tellurium dioxide (in excess) and vanadium pentoxide, The former method gives rise to yellow powders which readily hydrolyse is 0.ftTeO . and become colloidal with water. Their composition Na 2 2 6 or to the of 2V 2O 5 .#H 2O, where n=5, 10, according composition latter method is remarkable because the precipitating medium. The in acid or neutral all other heteropoly-acid salts are prepared either it rise to white the of which again solution ; gives needles, composition lose varies with the relative proportions of oxides used. The products some of their tellurium dioxide content on being recrystallised, and are 1 probably not true compounds. No vanado-tellurates have been prepared, although molybdo-, are known. tungsto-, phospho-, and other heteropoly-tellurates Vanado-iodates. When vanadium pentoxide is dissolved in hot of a soluble solutions of iodic acid, HIO 3 , orange-yellow crystals sparingly .I .4H are obtained. The substance having the composition V 2 5 2 5 2 four molecules of water are a factor in the constitution of the acid, since Several they cannot be removed without decomposition taking place. 2 viz. other similarly constituted compounds have been prepared, .5H 2V .3I O .1SH 0. These all V 2 5.2I 2 5 .10H 2 ; V 2 5 J 2 5 2O ; 2 5 2 5 2 on undergo hydrolysis with precipitation of vanadium pentoxide being boiled with water. By the action of ammonium hydrogen iodate, on the acid and iodate, ) 2 , NH 4H(I0 3 ) 2 , potassium hydrogen KH(IO 3 salts have been which have the V 2 5J 2 5.4H 2 S prepared following . The vanado- : O.V .2I 5 and K 2O.V 205.2I 2 5 compositions (NH 4 ) 2 2 5 2 " . The iodic acids are probably derivatives of ortho-iodic acid," I(OH) 5 has the compound V 2O5 .I 2O5 .4H 20, therefore, following probable 3 3 /V0 constitution : I<(X (OH) 4 Vanado-periodates. Vanadium pentoxide dissolves' in boiling solutions of the alkali periodates to produce yellow or red, crystalline to the vanado-periodates, the compositions of which approximate

: 5Na -'OH O and 5K O.5V .2l .a?H O. following 2O.fF 2 5 .2liO 7 2 2 2 5 2 7 2 various other By altering the relative acidity of the solution, complexes

4 . have been obtained, 5Na 0.7V O .I 7 .27H 2 ; 6Na 20.9V 2O5 e.g. 2 2 5 2 O I 3Na 0.2V .I .7H ; 3K 0.2V O J ; 2 7 .48H 2 ; 2 2 5 2 7 2 2 2 5 2 7 .17|H 2 .I O. The water in the last three com- and 3(NHJ 20.2V 2O5 2 7 .6JH 2 pounds has been shown not to be water of constitution, and the salts can be regarded as tribasic vanado-periodates of the general formula Y i 3 H: two atoms in the ion R3r r7 ^T^ 2 ; oxygen periodate [I0 6] radicals. 5 have been replaced by (VO 3 )' Oxalo-vanadates. These compounds are obtained in large action of yellow prisms (side by side with red polyvanadates) by the vanadium pentoxide on boiling solutions of alkali oxalates, or by the action of normal alkali vanadates on acid oxalates. They have the

24 i.e. general formula 3R' 2O.V2 5.4C 2 3.#H 2 or R 3 V w.H 2O,

1 der Jena, Dissertation (Giessen, 1907) ; Abegg and Auerbach, Handbuck anorganisclien Chemie (Leipzig), 1921, 4, i (second half), 1012. 3 Ditto, Compt. rend., 1886, 102, 756, 1019, 1115. 3 Bosenheim and Yang, Zeitsch. anorg. Ckem., 1923, 129, 181. 4 6 Butzback, Dissertation (Bern, 1905). Bosenheim and Yang, kc. cit. COMPOWDS OF VANADIUM. 91

can be looked as derivatives of they upon orthovanadic acid, H 3VO 4. The 1 ammonium, sodium, potassium and barium salts have been prepared. On boiled being with vanadium pentoxkle these salts give rise to the 2 oxalo-molybdovanadates, which also form bright yellow crystals. Those which most separate easily have the compositions : 2R' O.V .2Mo0 .o?H 2 A*2Cp 3 3 2 (R=NH 4 or K.) and

8R' iO.V a .2C .6Mo0 .a?H O. or 8 a 8 8 a (R=NH 4, Na, K JBa. ) These are most probably derivatives of the oxalo-vanadates in which co-ordinative atoms have oxygen undergone substitution by (Mo0 4 )" groups. Hence they are alternatively represented as

H0 and respectively.

Vanado-stannates are referred to on p. 67.

All the foregoing heteropoly-compounds contain vanadium pentoxide. A large number of other compounds have been prepared which, from their analytical data, appear to contain one of the lower oxides in varying proportions together with the pentoxide, e.g. vanadous- 3 4 vanadophosphates, vanadous-vanadoarsenates, vanadous-vanadomolyb- Q dates* vanadous-vanadotungstates. The constitutions of these have not been established.

PERVANADIC ACID AND THE PERVANADATES.

Pervanadic Acid has not been definitely isolated in the solid state. When vanadium pentoxide is dissolved in hydrogen peroxide solution which also contains sulphuric acid, a red solution is formed from which on concentration in vacuo greyish-yellow crystals, which are assumed to have the are composition HVO 4 , pervanadic acid, deposited. They give the reactions of a per-acid, but are difficult to analyse in conse- 7 quence of their unstable character. The preparation of the crystals is best carried out at low temperatures, because at ordinary temperatures vigorous evolution of oxygen takes place in consequence of the catalytic decomposition of the hydrogen peroxide by the vanadium pentoxide. A similarly coloured solution with similar properties is also obtained by 8 to the action of sulphuric acid on barium pervanadate. According Meyer 9 and Pawletta, the red solution contains the compound (VO 2 ) 2 (SO 4 ) 3, which undergoes hydrolysis in the following manner :

1 368 ; 1896, 225 ; Rosenheim, Ber., 1893, 26, 1191 ; Zeitsch. anorg. Chem., 1893, 4, u, 1 rend,., 1886, 102, 1019. Rosenheim, Itzig, and Koppel, ibid., 1899, 21, ; Ditte, Compt. 2 ffoMd&wcA 1908, Abegg and Auerbach, Joe. oft., p. 1040; Gmelin-Kraut, (Heidelberg), 3, ii, 204-205, 1094-1095. Gmelin-Kraut, loc. cit., pp. 127-128, 148. Ibid., pp. 629-630, 632-633. Ibid., pp. 203-204. Ibid., pp. 186-188, 634. Softener, Zeitsch. anorg. Chem., 1898, 16, 284. 173 J. Russ. Chem.Soc., Pissarjewsky, Zeitsch. physical Chem., 1903, 43, 160, ; Phys. 1902,34,210,472; 1903,35,42. fl 15 Zeitsch. Chem., Meyer and Pawletta, Zeitsch. anal Chem., 1926, 69, ; angew. 270. 1926, 39, 1284 ; Hothersall, J. Soc. Chem. Ind., 1924, 43, 92 VANADIUM, NIOBIUM, AND TANTALUM.

. (V0 4 ) t (S0 4) 8 +6H,0^3VO t(OH),+8H0 4 Reddish-brown. Yellow.

The can be looked as yellow compound VO 2 (OH) 3 thus formed upon a hydrated pervanadic acid, HV0 4.H 20, or as peroxyorthovanadic acid,*

HJ V - It decomposes slowly at ordinary temperatures, and more L. Q 3 ~J rapidly on being warmed, with evolution of oxygen and reprecipitation of vanadium pentoxide. It has been shown that in the formation of the red compound in solution each molecule of vanadium pentoxide reacts with two molecules of hydrogen peroxide :

V 2 5 +2H 2 2 =2HV0 4 +H 20.

Pervanadic acid appears, therefore, to be formed from metavanadic acid, HV0 3 . This view of the reaction and the formula for pervanadic acid are further supported by titrating the red solution with caustic soda, when it is found that two molecules of caustic soda are required for each molecule of vanadium pentoxide. The monobasicity of the acid is confirmed by measuring the equivalent conductivities of solutions of potassium pervanadate. A solution of pervanadic acid evolves a mixture of oxygen and chlorine on the addition of acid it evolves hydrochloric ; oxygen slowly on standing, more rapidly on warming, and leaves a residue of vanadium pentoxide. Decomposition of the solution is accelerated by the presence 2 of dilute sulphuric acid. Pervanadates of the alkali metals have been prepared by the action of hydrogen peroxide on the alkali metavanadates metals also ; pervanadates of several heavy have been isolated by double decomposition with ammonium pervanadate. These salts vary from bright yellow to deep orange in colour, and are amorphous or micro-crystalline. They undergo hydrolysis in dilute solution, e.g. and evolve oxygen when platinised asbestos is introduced into their concentrated solutions. The following metapervanadates have been 3 : prepared Ammonium pervanadate, NH 4V0 4 ; barium pervanadate, cadmium calcium Ba(V0 4 ) 2 ; pervanadate, Cd(V0 4 ) 2 ; pervanadate, lead lithium Ca(VO 4 ) 2 ; pervanadate, Pb(V0 4 ) 2 ; pervanadate, LiVO 4 ; silver sodium potassium pervanadate, KV0 4 ; pervanadate, AgV0 4 ; pervanadate, NaVO 4 ; and strontium pervanadate, Sr(V0 4 ) 2 , Scheuer was unable to prepare the pervanadates of aluminium, cobalt, copper, magnesium, manganese, and nickel. Pyropervanadates. Pyropervanadic acid has not been isolated, 4 but ammonium and potassium salts derived from it are known. The ammonium salt has the and is composition (NH 4 ) 4V 4Oi 1 , prepared by ammonium in dissolving metavanadate, NH 4V0 3 , an aqueous solution of hydrogen peroxide, adding ammonium hydroxide until the solution smells of ammonia, and then precipitating the salt with alcohol. The reaction is thought to take the following course : Ammonium meta-

1 Meyer, Zeitsch. anorg. Chem., 1927, 161, 321. 2 Auger, Gompt. rend., 1921, 172, 1355. 3 loc. cit. loc. cit. Scheuer, ; Pissarjewsky, ; Zzitsdi. anorg. Chem., 1902, 32, 341. * Mel&off and Pzssarjewsky, Ztitsch. anorg. Chem., 1899, 19, 405. COMPOUNDS OF VANADIUM. 93

is first pervanadate, NH4V0 4, formed, and this reacts with water to

ammonium in : yield pyropervanadate, (NH 4 ) 2H 2V 2 9 , solution O 00 00 NH 4 \/ 4 2NH V X \//ONH 4O.V=0+H 20= > V V < HO/ \O

This substance undergoes further change with ammonium hydroper- oxide, NH 4O.OH, which is also formed in solution at the same time,

to the : yield compound (NH 4 ) 4V 2 U 00 O O O O NH NH 4 X\\/ \X/ONH 4 = 40.\X \/ />NH 4 " y o V < > V V < "HjO/ \OjET NH 40.0/ \O.ONH 4

HO|ONH 4 NH 40;OH +2H 3O.

r Ammonium pyropervanadate crystallises in minute, slender, yellow , rhombic prisms, remaining undecomposed for some time when dry, then slowly decomposing with evolution of oxygen. With concentrated sulphuric acid it yields ozonised oxygen, and with dilute sulphuric acid hydrogen peroxide. The ammonium and potassium salts of orthopervanadic acid have 1 also been prepared. V0 is Orthopervanadates. The ammonium salt, (NH 4 ) 3 6.2|H2O, obtained by dissolving ammonium metavanadate in excess of concentrated ammonia solution, cooling to C., adding 30 per cent, hydrogen peroxide, and then precipitating with alcohol. The product, after washing with ether and drying, is pale blue, and probably has the constitution NH 0,0 4 V

salt which With large excess of hydrogen peroxide an indigo-blue is obtained. contains more oxygen, probably V0 2 (OH)(O.ONH 4 ) 2, is also and is Potassium Orthopervanadate, K 3V0 6 .2JH 20, blue, prepared similarly to the ammonium salt. to Vanadium oxyfluorides also react with hydrogen peroxide yield 2 as in the case of complex compounds. These are not so well defined and tantalum. the peroxyfluorides of niobium

VANADIUM AND SULPHUR. Vanadium Monosulphide, VS. The lowest oxide of vanadium which can be obtained by the reducing action of hydrogen is vanadium the . case of the however, reducing trioxide, V 2 3 In the sulphides, action of hydrogen appears to proceed still further, notwithstanding at the fact that hydrogen sulphide is a much less stable compound over heated a red heat than water-vapour. By passing hydrogen S at 1100 to 1200 C, for from four to eight vanadium trisulphide, V2 3,

1 Melikoff and Jelhchaninoff, Ber., 1909, 42, 2291. 2 Chemie 1907, in, 745, 781. Abegg, Handbuch der anorganischen (Leipzig), 3, 94 VANADIUM, NIOBIUM, AOT) TANTALUM. 1 Wedekind and Horst, 2 days, the monosulphide, VS, has been obtained. however, were unable to obtain any of the sulphide by this method, but found that it resulted, more or less impure, from the action of hydrogen Vanadium forms sulphide on hypovanadous oxide, VO. monosulphide or a brownish-black, either glistening, black scales, of density 4-2, has been amorphous powder, density 4-4. Its magnetic susceptibility 3 with studied. On being heated in air it absorbs oxygen readily dioxide. formation of vanadium pentoxide and evolution of sulphur both It is attacked only very slightly by boiling hydrochloric acid, acid hot concentrated strong and dilute, and boiling dilute sulphuric ; while nitric or dilute, sulphuric acid dissolves it slowly, acid, strong attacks it readily. It is soluble in colourless ammonium sulphide, giving a wine-red a purple solution, and in yellow ammonium sulphide, yielding solution. Caustic soda and ammonium hydroxide have a slight solvent action on the sulphide.

. convenient method for Vanadium Trisulphide, V 2S 3 The most in consists in the preparation of vanadium trisulphide quantity several hours over drawing the vapour of carbon disulphide for 4 results from the action vanadium pentoxide heated to redness. It also vanadous V or on the of hydrogen sulphide gas on oxide, 2 3 , higher oxides of vanadium, or from the action of hydrogen sulphide at a red heat on any chloride or oxychloride of vanadium. Vanadium trisulphide has been obtained in black, glistening scales, of density 3-7, or as a black, amorphous powder, of density 4-0, according to the initial material. On being heated in air this sulphide evolves sulphur dioxide and forms less than vanadium pentoxide, but the reaction proceeds much readily in the case of the lower sulphide. The behaviour towards acids, colour- caustic and less ammonium sulphide, yellow ammonium sulphide, soda, ammonium hydroxide is like that with the lower sulphide.

. is Vanadium Pentasulphide, V 2S 5 Vanadium pentasulphide prepared by heating vanadium trisulphide with slight excess over the at C. for several hours excess of calculated quantity of sulphur 400 ; This reaction is sulphur is finally removed with carbon disulphide. reversible, for on being heated in the absence of air the pentasulphide loses sulphur and re-forms the trisulphide. Heated in air it forms vanadium pentoxide. It is a black powder, of density 3*0. It does not its acids colourless differ from the lower sulphides in behaviour towards ; ammonium sulphide also dissolves it to give the purple solution given by the other sulphides, but yellow ammonium sulphide produces a brownish-red solution. It differs markedly from the other sulphides in being readily dissolved by caustic soda, especially on warming, and in this respect acts in a manner comparable to vanadium pentoxide. 5 Several thiovanadates analogous to the vanadates are known. The order of stability of the sulphides of vanadium is not the same series the as that of the oxides ; for whilst in the oxygen penta-compound is stable at a red heat, in the sulphur series the penta-compound is converted into the trisulphide at this temperature. Again, the trioxide is permanent in hydrogen at intense redness, whilst the trisulphide is reduced to the monosulphide under similar conditions.

1 Kay, J. Chem. Soc., 1880, 37, 734. 2 Wedekind and Horst, Ber. 9 1912, 45, 262. 3 Wedekind, Zeitsck. angew. Chem., 1924, 37, 87. * 5 Kay, loc. cit, Kay, toe. cit. COMPOUNDS OF VANADIUM. 95

A sulphide in which the vanadium is tetravalent, corresponding to the oxide VO has not hitherto 2 , been prepared, Vanadium Oxysulphides. No vanadium oxysulphides of definite composition have been prepared. By the action of acids on solutions of vanadium pentoxide in ammonium sulphide, or on solutions of alkali vanadates which have been saturated with hydrogen sulphide, brown precipitates are obtained which consist of oxysulphides of variable 1 composition. is Hypovanadous Sulphate, VSO 4.7H 2O, obtained in solution when vanadium pentoxide is dissolved in sulphuric acid and reduced in the absence of air with sodium-amalgam, zinc, or cadmium. The separation of the hydrate is effected by first reducing the pentoxide in acid solution with sulphur dioxide to the blue tetravalent state, and then continuing the reduction electrolytically in an atmosphere of carbon dioxide until the colour of the solution changes to violet. 2 Concentra- tion in vacua deposits transparent, reddish-violet crystals, which become bluish-violet even with traces of oxygen. The crystals are monoclinic, and isomorphous with crystals of ferrous sulphate, FeS0 4.7H 2Q. They undergo oxidation rapidly on exposure to air and are readily soluble in water dilute solutions ; aqueous in the absence of oxygen evolve 3 hydrogen, while concentrated solutions also yield hydrogen sulphide. When brought into contact with other compounds the sulphate exerts the same reducing action as hypovanadous chloride. Its solutions have the property common to solutions of ferrous sulphate and chromous sulphate of absorbing nitric oxide. Hypovanadous sulphate gives rise to double sulphates with the sulphates of ammonium, potassium, and rubidium. Hypovanadous am- is formed monium sulphate, VS0 4.(NH 4 ) 2S0 4.6H 20, when ammonium vanadate is electrolytically reduced in the presence of sulphuric acid. is Hypovanadous potassium sulphate, VSO 4.K 2S0 4.6H 20, made by electrolytically reducing vanadium pentoxide in sulphuric acid solution, then adding potassium sulphate and continuing the reduction. In the same way hypovanadous rubidium sulphate has been prepared, but it has not been found possible to obtain it free from vanadium rubidium alum. These double sulphates form violet, monoclinic crystals, which are not stable in air; they are not so rapidly oxidised as the simple hypovanadous are less soluble in water than the and salt, however ; they simple salt, give yellow or brownish-yellow solutions which may contain complex ions. Hypovanadous sulphate also has the property of forming mixed chromous these have crystals with magnesium, ferrous and sulphates ; .7H O.4 the formula: (V.Mg)S0 4.7H 20; (V.Fe)S0 4.7H 2 ; (V.Cr)SO 4 2 is Vanadous Sulphate or vanadium sesquisulphate, V 2 (S0 4 ) 3, readily obtained in solution by reducing a solution of vanadium pent- 5 oxide in sulphuric acid either by means of magnesium or electrolytically. The solid has been obtained by slowly heating to 180 C. in an atmosphere

. of carbon dioxide a solution of green acid vanadous sulphate, V 2 (SO 4 ) 3 in has been acidified H 2SO 4.12H 2 (see next page), water which slightly

vanadous , can with sulphuric acid. Ammonium sulphate, (NH 4 )V(S0 4 ) 2

1 1 loc. cit. Berzelius, Pogg. Annakn, 1831, 22, ; Kay, 2 Conant and Cutter, J. Amer. Ghem. Soc., 1926, 48, 1019. 3 Rutter, Dissertation (Leipzig, 1906). 4 55, Piccioi, Zeitsch. anorg. Ghem., 1899, 19, 204 ; Piccini and Marino, ibid., 1902, 32, 5 Butter, Zeitsch. Elektrochem., 1906, 12, 130. 96 VANADIUM, NIOBIU3V1, AND TANTALUM.

1 also be used. Vanadous sulphate forms a micro-crystalline, yellow in water it on heated in powder, which is insoluble ; decomposes being 2 air with formation of vanadium pentoxide. By the action of con- or centrated sulphuric acid on vanadic acetate several green yellowish- 3 green hydrates of vanadous sulphate have recently been prepared : .10- .4H V .9H ; V 2 4 ) 3 V 2 (S0 4 ) 3.3H 2 ; V 2 (S0 4 ) 3 2 ; 2 (S0 4 ) 3 3 (S0 tri- and to 11H 2O ; of these the tetra-hydrates appear correspond of chromium in general behaviour to the green tri- and tetra-hydrates sulphate. Electro- Acid Vanadous Sulphates and double vanadous sulphates. has two lytic reduction of solutions of vanadyl sulphate, VOSO 4 , given to the amount green, crystalline, acid vanadous sulphates, according of sulphuric acid also added : .4H or V .H SO .8H 0.4 4 ) a 2 2 (S0 4 ) 3 2 4 2 HV(S0 5 .6H or V .H SO 4.12H 2O. HV(S0 4 ) 2 2 2 (SO 4 ) 3 2 metavanadate in By electrolysing a solution of ammonium sulphuric of these acid acid instead of vanadyl sulphate, the ammonium salts viz. .4H and NH .6H 0. sulphates are obtained, NH 4V(SO 4 ) 2 2 4V(S0 4 ) 2 2 The latter is referred to again below. These acids (and their salts) differ of from one another in their general properties, for instance in the acidity their solutions, in the electrical conductivity of their solutions, and in the ease with which they undergo oxidation. From a study of their analogy with the corresponding compounds of chromium, the constitution has been ascribed to the acid, and [V(S0 4 )(H 2O) 4]S0 4H tetrahydrate to the acid* the constitution ) 2 2 2 hexahydrate [V(SO 4 ? (H 0) ]H.4H ^ By the action of sulphuric acid in varying quantity on solutions of the com- vanadic acetate in glacial acetic acid, compounds having

.5H and .8H have been ; positions HV(S0 4 ) 2 2 HV(S0 4 ) 2 2 prepared rise to .2H and . dehydration of these gives HV(S0 4 ) 2 2 HV(SO 4 ) 2 .5H O or The ammonium salt of the pentahydrate acid> NH 4V(SO 4 ) 2 2 has also been isolated. [V(H 2O) 4](S0 4 )(S0 4NH 4 ).H 20, When the reduction of the vanadyl sulphate is allowed to proceed of two in the presence of the sulphates of the alkali metals or thallium, series of double salts are obtained, the composition of which again varies with the amount of sulphuric acid present and with the other conditions.

.12H or S0 . Series I. has the general formula RV(S0 4 ) 2 2 R 2 4 to the of the alums V 2 (S0 4 ) 3.24H 20, which corresponds composition formed by the metals of Group III. and other groups. The vanadium alums are very similar in their behaviour to ferric, chromium and aluminium alums. They crystallise in regular, pentagonal hemihedra, varying in colour, with increase in atomic weight of the alkali metal, from the violet of the ammonium alum to the ruby-red of the caesium alum. The following table includes those alums which have hitherto 7 been prepared :

1 Auger, Compt. rend., 1921, 173, 306. 2 Stahler and Wirthwein, Ber., 1905, 38, 3978. 3 Meyer and Markowitz, Zeitsch. anorg. Ghem., 1926, 157, 211. 4 Trans. Amer. Electrochem. Brierley, /. Chem. Soc., 1886, 49, 823 ; compare Fischer,

Soc. t 1916, 30, 191. 5 6 Stahler and Wirthwein, loa. cit. Meyer and Markowitz, toe. cit. 7 Chem. Piocini, Zeitsch. anorg. Ohem., 1896, n, 106 ; 1897, 13, 441 ; Locke, Amer. J., loc. cit 1200. 1901, 26, 166; Auger, ; Eichner, Qompt. rend., 1927, 185, COMPOUNDS OF VANADIUM, 97 VANADIUM ALUMS.

Bultemann 2 observed that vanadium ammonium alum separates out in blue crystals from a solution containing sulphuric acid, but from solutions containing a weak acid, or from neutral solutions, red crystals are obtained. (The chromium alums can also be prepared in differently coloured modifications.) The analytical data, melting-point, electrical conductivity, rate of efflorescence, and general behaviour of both kinds con- of crystals are identical, so that it is difficult to ascribe different stitutions to them. Meyer and Markowitz 3 have shown that both acid forms separate out when the molecular proportion of sulphuric in the solution is less than that theoretically required, and attribute

O , or the red colour to the presence of traces of vanadous oxide, V 2 3

. and vanadium ctcsium its hydroxide, V(OH) 3 Vanadium rubidium alums behave in the same way. A vanadium guanidine alum has also 4 been prepared. alums is The general constitutional formula for the vanadium which differs from the formulae for other [V(H 4 2 ) 6](SO 4 )(SO 4R), are not attached double vanadium sulphates in that the sulphato groups to the nuclear vanadium atom. of the acid . Series II. This appears to comprise the salts HV(SO 4 ) 2 can be obtained this acid 6H 2O mentioned above, since they by boiling two members of the class with the respective alkali sulphates. Only have been examined : .6H O. .V .12H or 4 4 ) 2 2 (NH 4 ) 2S0 4 2 (S0 4 ) 3 2 (NH )V(SO or .6H 20. (Rb) 2S0 4.V 2 (S0 4 )3.12H 2 RbV(SO 4 ) 2 have been obtained By heating the alkali vanadyl sulphates, compounds of members which can be looked upon either as the dehydrated forms mentioned of this series, or as salts of the acid sulphate, HV(SO 4 ) 2, 5 above : .V . or ) 2S0 4 2 (S0 4 ) 3 (NH 4 )V(S0 4 ) 2 (NH 4 . ! or Na S0 4.V 2 (SO 4 ) 3 NaV(S0 4 ) 2 2 . or K S0 4.V 2(S0 4 ) 3 KV(S0 4 ) 2 2

These have the general formula [V(SO 4 ) 2]R. 141. 1 At 10 a Bultemann, Zeitsch. Ehktrochem., 1904, 10, 4 1925, 611, a Meyer and Markowifcz, Zoc. cit. Canneri, Gazzetta, 55, 78. 5 Rosenheim and Mong, Zeitsch. anorg. Ghern., 1925, 148, VOL. VI. : in. 98 VANADIUM, NIOBIUM, AND TANTALUM.

It is of some interest to note that iron and aluminium also give rise to double sulphates in which the number of molecules of water is less than in the alums. Vanadyl Sulphites. Normal sulphites containing tetravalent vanadium have not hitherto been prepared in consequence of the readiness with which hydrolysis takes place. A considerable number of vanadyl sulphites are, however, known, as well as their double salts, which, no doubt, are heteropoly-acid compounds. from the When a mixture of the oxides V 2O 3 and V 2 4 , resulting decomposition of ammonium metavanadate, is treated in boiling aqueous solution with sulphur dioxide, a blue liquid is obtained from which under suitable conditions the following blue, crystalline hydrates of

isolated : .3lI O VOS0 . can be ; vanadyl sulphite, VOS0 3 , VOS0 3 2 3 also rise to an 4H 2O ; VOS0 3 .SH 2O. The liquid gives amorphous, green, hydrated, basic vanadyl sulphite, V0 2.VOS0 3,4jH 2 or 1 2Y02.SO2.4JH20. Another blue, crystalline complex sulphite, having a similar the composition 4VO 2.3S0 2 .10H 20, has been prepared by 2 process, and by reducing barium metavanadate in aqueous suspension, results. 3 the blue, crystalline compound 3VO 2.2S0 2 .4|-H 2 Double Salts of Vanadyl Sulphite. By the action of sulphur dioxide on vanadates in the presence of varying molecular proportions of alkali sulphites, two series of alkali vanadyl sulphites have been obtained. Series I. This consists of blue compounds having the general for- .ti?H as mula R 3O.3YO 2.2S0 2 2O, which can be regarded derivatives of the vanadyl sulphite, 3YO 2.2S0 2 .4|H 2O, which can be alternatively written, H 20.3VO .2S0 2.3|HoO. The following have been prepared : (NH 4 )oO.

3V0 2.2SO;.H 2 ; Na 20.3VOo.2S0 2.3H 2 ; K 20.3V0 2.2S0 2 ; and ZnO.3VO 2.2S0 2 . Series II. This comprises green substances which are generally less stable than the members of former series, and have the general formula

R' O. : 2O.VO 2 .2S0 2^H 2 The following are known

S0 .VOSO .1 or 2 or .1 or (NH 4 ) 2 3 3 H,0 (NH 4 ) 2O.V0 2.2S0 2 2H 2O.

Na aS0 3.VOSO 3 .5H 2O or Na 2O.V0 2.2S0 2,5H 20.

K 2SO 3.VOSO 3 .2H 2O or K 2O,V02 .2S0 2.2H 20. 4 In addition to the foregoing, Gain has prepared a large number of alkali vanadyl sulphites in which the proportions of basic oxide, vanadium dioxide, and sulphur dioxide vary considerably :

2(NH4 ) 20.10V0 2 .7S0 2.16H 2O. 2K 20.6V0 2 .5S0 2 .5H 20.

(NH 4 ) 20.6VO 2.4S0 2.4H 2O. K 20.4VO 2.3S0 2 .2HoO. 5Li .6S0 20.2VO 2 2 .8H 2O. K 2O.6VO .4S0 2.5H 2O. Na 0.4V0 .3S0 0. 2 2 2.4H 2 2Rb 20.4V0 2.3S0 2.2H 2O. Na OJOV0 .6SO .2H 0. 2 2 2 2 Rb 20.2V0 2.3S0 2.4H 2(X 3Cs 2O,2V0 2.4SO 2 .SH 2O.

2T1 2O.2V0 2 .3S0 2.4H 2O.

T1 20.6VO 2.4S0 2 .SH 20. It is improbable that these are all true individual chemical compounds.

Vanadyl Sulphates. By treating hypovanadic oxide, VO 2 , with acid at sulphuric about 200 C., or by reducing a sulphuric acid solution 1 Gain, Ann. Chim. Phys., 1908, [viii], 14, 224. 2 Gain, Compt. rend., 1906, 143, 823. 3 and Zeitsch. Koppel Behrendt, anorg. Chem., 1903, 35, 154 ; Ber. 9 1901, 3929. 4 34, Gain, loc. cit. COMPOUNDS OF VANADIUM. 99 of vanadium vrith pentoxide, V 2O 5 , sulphur dioxide, two distinct com- are pounds obtainable, according to the quantity of sulphuric acid present : Neutral (a) vanadyl sulphate, VOSO 4 . Acid (b) vanadyl sulphate, 2VOSO 4.H 2SO 4.o?H 2O. exists Vanadyl Sulphate, VOSO 4, in two modifications, insoluble and soluble. The insoluble variety is best obtained by heating any of the acid to vanadyl sulphates 260 C. It is a greyish-green, micro-crystalline powder, which is insoluble both in hot water and in hot dilute acids. It is readily decomposed by alkalis to form brown, hydrated hypovanadic oxide. On being heated at 130 C. in a sealed tube with a little water, it undergoes conversion into the soluble form, which can also be obtained of a solution of by evaporation the hydrate, 2VOSO 4 . to 1 or 7H 2O, dryness, by evaporation of a solution of vanadium pentoxide 2 in sulphuric acid in the presence of alcohol. This modification is a blue, amorphous, gummy mass, completely soluble in water, giving a blue solution which becomes green in air and slowly deposits the green . heated in oxide, VO 2 On being the absence of air it again forms

oxide ; in the of air the is hypovanadic presence pentoxide, V 2O5 , produced. 3 4 Several blue, crystalline hydrates of vanadyl sulphate have been

from : prepared hypovanadic oxide, VO 2 VOSO 4.2H O ; VOSO 4.

.3H VOS0 . 2JH 2 ; VOS0 4 2 ; 4.3|H 2O ; VOSO^oEUO ; VOSO 4 .7H O. The 6JH 8O ; VOSO 4 2 dihydrate, VOSO 4.2H SO, has been pre- 5 pared electrolytically. Most of these give a greyish-green monohydrate, VOS0 4.H 20, on being heated to 150 C., and all of them undergo dehydration to form insoluble vanadyl sulphate at 2GO C. It is doubtful if these are all individual chemical compounds ; they may be mixtures of hydrates. They are, however, characterised by their crystalline forms, and in that they can, in several cases, be prepared by different methods. of Add Vanadyl Sulphates, general formula 2VOSO 4 .H 2SO 4.a?II 2O, are prepared by reducing solutions of vanadium pentoxide in sulphuric acid of such concentration that there are more than three molecules 6 of sulphuric acid per molecule of vanadium pentoxide. Evaporation on the water-bath yields the pentahydrate, 2VOSO 4.H 2SO 4.5H 2O, which, on being heated to 200 C., undergoes dehydration and forms the com- .3SO . at pound 2VOSO 4.SO 3 or 2VO 2 3 By treatment intermediate

also : . temperatures the following hydrates have been prepared 2VOSO 4 7 H 2S0 4.3H 20; 2VOSO 4.H 2SO 4.2H 2O ; 4VOSO 4.2H 2SO 4.H 2O. Gain has shown that hypovanadic oxide combines with sulphur trioxide to produce various other complexes all of which are blue, crystalline, fairly stable substances. They have the general formula 4VO 2 . trSO 3.2/H 2O, in which x varies between 4 and 10. Double Salts of Vanadyl Sulphate. Two series of double salts of

1 Crow, J. Ohem. Soc., 1876, 30, 457. 2 352. Guyard, Bull Soc. Mm., 1876, [ii], 25, 3 arid Zeitech. Garland, Ber., 1876, 9, 869 ; 1877, 10, 2109 ; Koppel BeKrendt, anorg, wn., 1903, 35, 170. 4 Gain, Ann. Ghim. Phys., 1908, [viii], 14, 224. 5 Fischer, Trans. Amer. Electrochem. Soc., 1916, 30, 193. 8 Compare Auger, Compt. rend., 1921, 173, 306. 7 1154. Gain, loc. cit. ; Compt rend., 1906, 143, 100 VANADIUM, NIOBIUM, AND TANTALUM.

1 vanadyl sulphate and alkali sulphates are known. The members of .R' the first series have the general formula 2VOS0 4 2S0 4.iTH 2O> and are obtained by reducing solutions of the alkali metavanadates in sulphuric acid with sulphur dioxide and then adding excess of the particular alkali sulphate desired. They form blue, crystalline compounds which undergo dehydration at 175 C. The following have been prepared :

.H S0 . 2VOS0 4.(NH 4 ) 2S0 4 2 ; 2VOS0 4.(NH 4 ) 2 4

. 4VOS0 4.2Na 2S0 4.5H 2 ; 2VOS0 4.Na 2S0 4 2VOS0 4.K 2S0 4 .

2 Fischer, by the same method, prepared the double sulphates with calcium, magnesium and aluminium sulphates. The members of the second series possess the general formula VOS0 4.R" 2SO 4.tTlI 20, and thus contain a smaller molecular proportion of vanadyl sulphate than the members of the preceding series, from which they are prepared by the prolonged action of alcohol on neutral solutions. They also yield dark blue, crystalline compounds, which undergo dehydration at 175 C. The following are known :

S0 .7H . 2VOS0 4.2(NH 4 ) 2 4 2 ; VOS0 4.(NH 4 ) 2S0 4

VOSO 4 .Na 2S0 4.4H 2 ; VOS0 4.Na 2S0 4 .

VOSO 4 .K 2S0 4.3H 2 ; VOS0 4.K 2S0 4 .

The general compositions of these two series of double salts correspond to the compositions of the double sulphates of divalent metals, e.g. 3 2ZnS0 4.K 2S0 4 and ZnS0 4.K 2SO 4.6H 2 0.

Vanadic Sulphates. Two of these are known, viz. V 2 5.3S0 3 and V 2 5 .2S0 3 . The former has been prepared in ruby-red, transparent with excess of octahedra by boiling vanadium pentoxide sulphuric acid ; if the boiling is continued for some time, golden-yellow needles separate out. The crystals absorb water rapidly and simultaneous formation of 4 5 vanadyl sulphate takes place, so that analysis is difficult. Miinzing doubts the existence of O .3S0 because V 2 5 3 , by the same process he obtained the substance V .2S0 which is also 2 5 3, extremely deliquescent, and exists either as a red, crystalline mass or as a yellow powder. If the sulphuric acid solution is strongly heated, a dark-brown modification is obtained which changes slowly into the red variety. It has recently been shown that when heated in sulphuric acid solution, tetravalent vanadium is partially oxidised and pentavalent vanadium partially 6 reduced to an equilibrium mixture. A crystalline form of the com- .2S0 with rather different pound V2 5 3 , properties, has been obtained 7 by the decomposition of alkali vanadyl sulphates. The action of concentrated sulphuric acid on vanadium pentoxide has also given a .3S0 .3H in either or 8 hydrate, V 2 5 3 2 9 yellow red, deliquescent crystals. This can be written hydrate alternatively V 2 5 .3H 2S0 4, but no corresponding salts are known, although by addition of the respective alkali

1 Koppel and Behrendt, loc. cit 2 Fischer, Trans. Atner. Mectrochem. Soc., 1916, 30, 187. 3 Rosenheim and Mong, Zeitsch. anorg. Chem., 1920, 148, 26. 4 Gerland, Ber., 1878, u, 99. 5 Munzing, Gkem. Zentr., 1889, ii, 908. 6 Eiehner, Gampt. rend., 1927, 185, 1200, 7 Bosenheim and Mong, loc. cit. 8 Ditte, Compt. rend., 1886, 102, 757. COMPOUNDS OF VANADIUM. 101

to its solutions sulphate two complex salts have been prepared, which can be looked as double of .SO upon sulphates V 2O 5 3 , namely, .2S0 .4I1 (NH 4 )AV 2 5 3 2 and K 2O.V 2O 5 .2S0 3 .6H 2O. A compound intermediate in composition between vanadic and sulphate vanadyl sulphate has been obtained by reducing a solution of a vanadic salt in sulphuric acicl with sulphur dioxide, or by mixing solutions of the 1 pentoxide and the dioxide in sulphuric acid. is Vanadyl Dithionate, VOS 2 6 , prepared by treating a solution of with barium vanadyl sulphate dithionate. It decomposes on standing, with evolution of sulphur dioxide. 2 Thiovanadates and Oxythiova?iadates. A series of crystalline salts is known which resemble the ortho- and pyro-vanadates in composition, that the is except oxygen either wholly or partially replaced by sulphur. Cu occurs Copper orthothiovanadate, 3 (VS 4 ) 2 , naturally as" suhanite (see p. 11). The following thio- and oxythio-compounds have been 3 prepared. Ammonium is Orthothiovanadate, (NH 4 ) 3VS 4, obtained in rhombic crystals by the action of ammonium hydrosulphide on potassium metavanadate or sodium pyrovanadate solution. It also separates out slowly from an ice-cold saturated solution of ammonium metavanadate in concentrated ammonium hydroxide which has been previously treated with hydrogen sulphide. Ammonium is Pyroxyhexathiovanadate, (NH4 ) 4V 2S6 O, deposited in red crystals when hydrogen sulphide is passed into a solution of ammonium metavanadate in ammonium hydroxide of higher specific gravity than 0-898 and the mixture kept for several months at a low The temperature. corresponding potassium salts, K 4V 2S 6O.SH 2O and K4V 2S 6 O.1|H 2O, are prepared in a similar manner. The normal has potassium orthothiovanadate, K 3VS 4 , not been separately isolated, although mixtures which most probably contain it, together with the normal ammonium salt, have been prepared., Sodium Orthoxytrithiovanadate, Na3VS 3O.5H 2O, is obtained in dark, reddish-brown, deliquescent crystals by the action of sodium hydrosulphide on sodium pyrovanadate. The anhydrous salt is formed when vanadium pentoxide, sulphur and sodium carbonate are fused 4 together, or by the action of hydrogen sulphide on sodium orthovanadate at 500 to 700 C. 5

Sodium Orthoxymonothiovanadate, Na 3VSO 3.10H 20, has been obtained in orange-yellow crystals by the action of sodium hydrosulphide on sodium pyrovanadate. The action of hydrogen sulphide at high temperatures on sodium pyro- rise to vanadate gives sodium pentathiopyrovanadate, Na4V 2S5O 2, which is a hygroscopic substance very similar in its colour and lustre to potassium permanganate. difficult to isolate Thiovanadates of the heavy metals have proved ; addition of solutions of salts of zinc, manganese, copper and silver to a solution of ammonium thiovanadate gives precipitates which consist

1 Auger and Eichner, Compt. rend., 1927, 185, 208. 2 Bevan, CJiem. News, 1878, 38, 294. 3 Kruss and Ohnmais, Ber., 1890, 23, 2547 ; Annalen, 1891, 263, 39. 4 Kruss, Zeitsch. anorg, Ghem. 9 1893, 3, 264. 6 Locke^ Amer. Chem. J. f 1898, 20, 373. 102 VANADIUM, NIOBIUM, AND TANTALUM. of sulphides or thiovanadates of the metals, or mixtures of both. A S black, crystalline lead salt of the probable composition Pb 2V 3 2O 5 , 1 lead dithiopyrovanadate, has, however, been obtained.

VANADIUM AND SELENIUM.

Vanadium Selenides. The preparation of the following selenides 2 of vanadium has been claimed in an American patent, but no details Se or analytical data are given : V 2Se (white), V 2Se 2 (yellow), V 2 3 (red), V 2Se 4 (dark blue), V 2Se5 (green), V 2Se8 , is in micro- Vanadyl Selenite, V0 2.Se0 2.2H 2O, obtained blue, is scopic crystals when the hydrate of hypovanadic oxide, V0 2.2H 20, dissolved in an aqueous solution of selenous acid and the solution 3 evaporated. Efforts to prepare normal selenates and selenate alums of trivalent vanadium were unsuccessful, but several hydrated acetoselenates of trivalent vanadium have been isolated. 4 Vanadyl Selenates. Two blue, deliquescent, crystalline vanadyl selenates, having the compositions 4VO 2.7Se0 3.14H 2 and 2VO 2 . of a solution 5Se0 3 .10H 20, have been obtained by very slow evaporation of in selenic acid, 5 hydrated hypovanadic oxide, V0 2.2H 20, Vanadium pentoxide and selenous acid condense together to form a large number of complex vanadoselenites which are described on p. 89.

VANADIUM AND CHROMIUM.

Normal chromates of vanadium have not been prepared. An ammonium O.V O .2Cr0 .7H has been vanadochromate, 2(NH 4 ) 2 2 5 3 2O, prepared in red crystals by dissolving vanadium pentoxide in ammonium chromate solution and evaporating at ordinary temperatures in vacua.

VANADIUM AND NITROGEN.

Vanadium combines slowly with nitrogen when heated in the gas to high temperatures. A substance having the empirical formula V 2N 7 was obtained at a red heat, but the existence of this compound must be regarded as doubtful, because the mononitride, VN, is formed at a 8 white heat. The temperature at which combination takes place is also somewhat uncertain. Pure vanadium does not absorb nitrogen 9 below 1250 C., but with a ferrovanadium alloy it was found that absorption of nitrogen by the vanadium takes place at an increasing rate from 800 to 1200 C., when a maximum absorption of 8 per cent, of nitrogen is reached; at higher temperatures the nitrogen content 10 decreases rapidly,

1 Locke, loc. cit. 2 von Oefele, U.S. Pat. 1154949 (1915). 3 Gain, Ann. Ohim. Phys., 1908, [viii], 14, 224. 4 Meyer and Markowitz, Zeitsch. anorg. Ohem., 1926, 157, 211. 5 loc. cit. also Gain, ; Compt. rend., 1907, 144, 1271. 6 Ditte, Compt. rend., 1886, 102, 1019, 1105. 7 Mutliinann, Weiss, and Riedelbauch, Annahn, 1907, 355, 58. 8 Whitenouse, J. Soc. Chem. Ind., 1907, 26, 738. 9 J. Russ. J. Shukoff, Phys. Chem. Soc., 1908, 40, 457 ; Ghem. Soc., Abs., 1908, [ill, 94, 484. 10 and Tchigevsky Blinow, Eev. Soc. russe de HetaU., 1914, i, 636 ; J. Soc. Chem. Abs. ., t 1916, 35, 893 ; compare also Tammann, Zeitsch. anorg. Ohem., 1922, 124, 25. COMPOUNDS OF VANADIUM. 103 Vanadium Subnitride, V 2N, is described as a black powder which is not attacked by hydrochloric acid or caustic potash solution. It is soluble in concentrated nitric and sulphuric acids, and evolves ammonia on being heated with solid caustic potash. Vanadium Mononitride, VN, can be prepared synthetically, but it is more conveniently prepared by calcining ammonium metavanadate, V0 in and NH 4 3 , air, heating the black residue to a white heat in a 1 current of dry ammonia. It has also been obtained by the action of ammonia on vanadium 2 dry gas oxytrichloride, VOC1 3 , and by heating a mixture of vanadous oxide, V2 3 , and carbon at 1200 C. in an 3 atmosphere of nitrogen. It is a greyish-brown powder which melts with decomposition at 2050 C. Its density is 5-91. The electrical conductivity, magnetic susceptibility, and crystal structure have been 4 investigated, and the dissociation pressures and calculated heat of formation are given by Slade and Higson.5 Vanadium mononitride is a stable it very compound ; yields vanadium pentoxide only on being very strongly heated in air. It evolves ammonia on being treated with 6 steam at 400 C., when heated with soda-lime or when boiled with caustic potash solution. Nitric acid dissolves it, but hydrochloric acid and sulphuric acid are without action. Vanadium is obtained as a Dinitride, VN 2 , black powder by ammonia over passing gas vanadium oxytrichloride, VOC1 3 , heated in a glass tube (to expel the ammonium chloride formed), washing with ammonium hydroxide, and drying in vacua over sulphuric acid. It oxidation evolves air undergoes and ammonia on being exposed to ; heated it evolves on being nitrogen and forms the mononitride, VN ; 7 it is readily attacked by molten caustic potash and hot nitric acid. Nitrites. Alkali Vanadyl By dissolving hypovanadic oxide, V0 2, in aqueous solutions of vanadyl nitrites the following alkali vanadyl nitrites have been prepared :

K 20.2V0 2.2N 2 3.4H 2O . . Colourless, regular, hexagonal prisms.

.2N .6H . Pale (NH 4 ) 2O.2VO 2 2O 3 2 yellow prisms.

8 The corresponding sodium salt is very unstable. Vanadium Nitrates. No nitrates of vanadium have been isolated. When hypovanadic oxide is dissolved in nitric acid the blue solution results contains but on which probably vanadyl nitrate, VO(N0 3 ) 2, evaporation oxidation ensues and hydrated vanadium pentoxide is obtained. Blue solutions of vanadyl nitrate are more conveniently obtained by precipitating vanadyl chloride with silver nitrate or 9 vanadyl sulphate with barium nitrate. Addition of nitric acid to hexanimino-vanadiuni trichloride yields liexammino-vanadium nitrate,

[V(NH 3 )6](N0 8 ) 3 .

1 Boscoe, J. Chem. Soc., 1870, 23, 344, 358. 2 Roscoe, ibid., 1868, 21, 349. 3 Friederich and Sittig, Zeitsch. anorg. Chem., 1925, 143, 303. * Wedekind and Horst, Ber. 9 1912, 45, 268 ; Friederich, Zeitsch. Physik, 1925, 31, 268. 813 ; Becker and Ebert, ibid., 1925, 31, 6 J. 215. Slade and Higson, B.A. Reports, 1913/451 ; Chem. Soc., 1919, 115, 8 Whitehouse, loc. cit. 7 134. Boscoe, J. Chem. tfoc., 1868, 21, 349 ; Uhrlaub, Pogg. AnnaUn, 1S58, 103, 8 Gain, Ann. Chim. Phys. t 1908, [viii], 14, 224. 9 Guyard, Bull. Soc. chim., 1876, [it], 25, 352. 10 Zeitsch. Chem. 177. Meyer and Backa, anorg. 9 1924, 135, 104 VANADIUM, NIOBIUM, AND TANTALUM. VANADIUM AND PHOSPHORUS. .H is a Vanadyl Hypophosphite, VO(H 2P0 2 ) 2 2O, bluish-green, solutions soluble, crystalline substance, obtained by heating sparingly 1 of vanadium pentoxide and hypophosphorous acid. .P .3H and 2V0 .3P Vanadyl Phosphates, V0 2 2 5 2 2 2 5 -1H20> the action of oxide on are prepared in blue needles by hypovanadic acid. 2 phosphoric . as a .30H 5 is obtained Vanadous Pyrophosphate, V 4 (P 2 7 ) 3 2 vanadium ammonium alum is added green, liocculent precipitate when 3 to a solution of an alkaline pyrophosphate. and The vanadophosphates, molyldovanadoplwspliates, tungstovanado- on 84 and 85. phospliates have been described pp. VANADIUM AND ARSENIC. Vanadyl Arsenates. Two of these have been prepared by the of arsenic acid : action of hydrated hypovanadic oxide on solutions .3As O .6H 0. are both sky-blue, 2VO a.2As 2 5 .8H 2 and 2V0 2 2 5 2 They the latter becomes on crystalline compounds, and rapidly green 4 exposure to air. and arsenic have been dealt The heteropoly-acicl salts of vanadium with on pp. 85 and 86. VANADIUM AND CARBON. or vanadium When either vanadium trioxide, V 2O 3, pentoxide, V is reduced with carbon in the electric furnace, the product contains 2 5 , to 25 carbon in proportions varying from 4 per cent, per cent., depending 5 If a mixture of on the temperature attained and other conditions. in the carbon tube of the the pentoxide and sugar charcoal is heated electric furnace for ten minutes, a definite carbide having the formula VC is obtained. More recently this compound has been prepared by heating a mixture of vanadium trioxide and carbon in an atmosphere 6 of hydrogen at 1100 C. Vanadium Carbide, VC, forms silvery-white crystals which are 5-25 to 5-40 2750 C. harder than quartz ; density ; m.pt. approximately Vanadium carbide is not attacked by water, hydrochloric acid, or it in the hydrogen sulphide, even at a red heat. Nitric acid dissolves cold. It burns at 800 C. in chlorine with incandescence, and at a red heat it reacts with oxygen, nitrogen, ammonia, potassium nitrate, and fused with potassium chlorate. On being vanadium trioxide, V 2 3 , in a crucible provided with a refractory lining, the carbon burns away 7 and a product containing 98-11 per cent, of vanadium is obtained. 8 The dissociation pressures, electrical conductivity, and crystal structure for this carbide 9 have been measured.

1 Mawrow, Zeitsch. anorg. Chem., 1907, 55, 147, 2 Gain, Oompt. rend., 1907, 144, 1271. 3 Rosenheim and Triantaphyllides, Ber., 1915, 48, 582. 4 1 3?riedheim and Berzelius, Pogg. AnnaUn, 1831, 22, ; Sckmitz-Dumont, Ber.,

1890, 23, 2600 ; Gain, Oompt. rend., 1907, 144, 1271. s Moisaan, ibid., 1893, n6, 1225 ; 1896, 122, 1297; Ruff and Martin, Zeitsch. angew. 18. Chem., 1912, 25, 49 j compare p. 6 Friederieh and Sittig, Zeitsch. anorg. Chem., 1924, 144, 172. 7 Buff and Martin, loc. cit. 8 Slade and Higson, /. Chem. Soc. t 1919, 115, 210. 9 Priederich, Zeitsch. Physik, 1925, 31, 813 ; Becker and Ebert, ibid., 1925, 31, 268. COMPOUNDS OF VANADIUM. 105

Moissan reported the existence of several other vanadium carbides, C in V 3C, V.jC 2 , V 2 2 , V 2C 3 , and some of these have received attention connection with the investigation of the constitution of vanadium steels. Arnold and Reed isolated a double carbide of iron and vanadium having the formula 2Fe C.V C but be a mixture of several other 3 4 3 , V 4C 3 may 1 carbides. The action of various etching agents on vanadium carbide has been investigated with a view to distinguishing it from other car- 2 bides in metallographic work. A carbide of vanadium is also obtained by the action of carbon monoxide on the metal at 500 to 800 C. The finely divided metal catalyses the reaction, 2CO=CO 2 +C, and the carbon thereby isolated is taken up by the vanadium. 3 Normal carbonates of vanadium are unknown. An unstable am-

of .7VO .5CO . monium vanadyl carbonate, composition 3(NH 4 ) 2CO 3 2 2 addition of 16H 20, has been obtained in small, violet crystals by the 4 ammonium carbonate to a neutral solution of vanadyl sulphate.

VANADIUM AND CYANOGEN. Vanadyl Cyanide. Berzelius 5 obtained vanadyl cyanide by the action of hydrocyanic acid on hydrated hypovanadic oxide in the absence of air. The product was not, however, analysed. is obtained Potassium Vanadocyanide, K4[V(CN)6 ].3H 2O, by acetic acid means reduction of a solution of vanadous oxide, V 2 3, in by treated of potassium amalgam in the absence of air, the product being 6 with potassium cyanide. Its aqueous solution is red, and addition of alcohol to it precipitates brownish-yellow, apparently tetragonal reduces crystals which rapidly become dark blue in moist air. It silver salts to metallic silver, mercuric chloride to mercurous chloride, and is analogous to the corresponding potassium ferro-, potassium mangano-, and potassium cobalto-double cyanides. is the Potassium Vanadicyanide, K 3[V(CN)6], prepared by a addition of excess of concentrated potassium cyanide solution to in concentrated solution of vanadous chloride, VC1 3 ; precipitation the cold with alcohol gives rise to small rhombohedral plates. The addition solution is not very stable and rapidly becomes turbid, while the V of an acid produces the green colour which is characteristic of to be unstable, ion. The complex ion [V(CN)6]"' appears, therefore, and unlike the corresponding [Fe(CN)6]'"> [Cr(CN)6]'", [Co(CN)6]'" complex ions. The solution reacts with salts of heavy metals to yield 7 variously coloured precipitates of double cyanides. An unstable double salt of potassium pyrovanadate and potassium has been 8 as well as a com- cyanide, K 4V 2O 7 .4KCN.14H 2O, prepared, plex addition compound of sodium vanadyl cyanide and hexamethylene- .2C .5H O. 9 tetramine,Na 3VO(CN)5 6H12N 4 2

Arnold and Keed, J. Iron Steel Inst., 1912, 85, 215. 569. Groesbeck, Proc. Amer. Soc. for Testing Materials, 1926, 26, i, Meyer and Backa, Zeitsch. anorg. Chem., 1924, 135, 177. Koppel, Goldmann, and Kaufmann, ibid., 1905, 45, 349. Berzelius, Pogg. Anwkn, 1831, 22, 1. 342. Petersen, Ber., 1903, 36, 1911 ; Zeitsch. anorg. CJiem., 1904, 38, Locke and.Edwards, Amer. Chem. J., 1898, 20, 594. Petersen, toe. cit. Barbieri and Parisi, er., 1927, 60, [B], 2418. 106 VANADIUM, NIOBIUM, AND TANTALUM. Vanadium Ferrocyanides. Solutions of vanadates, when treated 1 with potassium ferrocyanide, yield a precipitate of doubtful composition. The compound is insoluble in mineral acids of high concentration, and a method involving its precipitation has recently been suggested for the estimation of vanadium in steels. 2 Double Salts of Vanadous Thiocyanate. When vanadous sulphate, V in is treated with barium a 2 (SO 4 ) 3 , solution, thiocyanate, green 3 solution is obtained which is believed to contain vanadous thiocyanate, but to isolate this have not met with V(SCN) 3 , attempts compound success. Its double salts with the alkali thiocyanates have, however, been obtained in well-defined crystals when an alkali thiocyanate is substituted for the barium salt. The following are known : dark Ammonium vanadous thiocyanate, 3NH 4SCN.V(SCN)3.-iH 2 ; green crystals. red tablets Sodium vanadous tlviocyanate, 3NaSCN.V(SCN) 3 .12H 2 ; or leaflets. .4H Potassium vanadous thiocyanate, 3KSCN.V(SCN) 3 2O ; bright red crystals. contain a anion These compounds probably do not complex [V(SCN) G] ; if they do, it must be extremely unstable in dilute solution, because (a) their aqueous solutions, which are brown at first, assume the green colour due to the V"* ion ; (b) addition of an alkali precipitates vanadous acidified solutions the addition of ferric hydroxide, V(OH) 3 ; (c) on salts give the blood-red colour characteristic of the (SCN)' ion; (d) 4 depression of freezing-point data bear out the foregoing facts.

Hexa-metliylenetetramine aquo-vanadous pentatliiocyanate, (C 6 H12N 4.H) 2 [V(SCN) 5.(OH) 2].H 20, and a basic pyridine vanadous tetrathiocyanate, from (C5H5N.H) 3.[V(SCN) 4 (OH) 2], have, however, been prepared 5 ammonium vanadous thiocyanate. Ammonium Vanadyl Thiocyanate. This compound and the corresponding potassium vanadyl thiocyanate are obtained in blue, isomorphous, rhombic crystals by the addition of ammonium or potassium thiocyanate, as the case may be, to a neutral solution of vanadyl sulphate, VOS0 4 . Their compositions are 2NH 4SCN.VO(SCN) 2 .5H 2O and2KSCN.VO(SCN) 2.5H 20. Neither the corresponding sodium com- nor the free has 6 pound vanadyl thiocyanate, VO(CNS) 2 , been isolated, but a violet-red bismuth 2Bi .3VO .7H O vanadyl thiocyanate, (SCN ) 3 ( SCN ) 2 or 7 (VO) 3[Bi(SCN) G ] 2.7H 20, has recently been prepared. Among other organic compounds of vanadium the following are known: Vanadous acetate* vanadous acetylacetonate, vanadous benzoyl- 9 11 acetonate, vanadow carbamides vanadyl acetonates, vanadyl acetate,

1 Wyrouboff, Ann. Chim. Phys., 1876, [v], 8, 444 ; compare Atterberg, Ber., 1876* 9, 1475. 2 Evans and Clarke, Analyst, 1928, 53, 475. 3 Solletino J. Compare Bongiovanni, chimico farmaceutico, 1910, 49, 467 ; CJiem. Soc., A, 1910, 98, [i], 721. 4 foe. cit. Locke and Edwards, ; Cioci, Zeitsch. anorg. C%em., 1899, 19, 308. 5 flcagliarini and Tartarini, Gazzetta, 1923, 53, 139, 876. c Koppel and Goldmann, Zeitsch. anorg. CJiem., 1903, 36, 381. 7 Paciello and Foa, Gazzetta, 1923, 53, 526. 8 Meyer and Markowitz, Zeitsch. anorg. Chem., 1926, 157, 211. 9 Morgan and Moss, /. Chem. Soc., 1913, 103, 85. 10 Barbieri, Am R. Accad, Lincei, 1915, [v], 24, 435. 11 he. cit. Morgan and Moss, ; Parisi, Gazzetta, 1927, 57, 859. COMPOUNDS OF VANADIUM. 107 vanadyl oxalate, vanadyl malonates, vanadyl succinale, vanadyl salicylate* alkali vanadous oxalaies? alkali vanadyl oxalates? alkali vanadyl tar- 1 trates, alkali vanadyl salicylates, alkali vanadyl citrates? oxalo-vanadatesf 8 citro-vanadates? complex cinnamates and camphorates, aceto-selenates?

VANADIUM AND SILICON.

Vanadium and silicon are miscible in all proportions in the liquid state up to 60 per cent, vanadium. The eutectic point is at 1411 C. near to the silicon end of the series, while the composition at the 10 to the . maximum approximates composition of the disilicide, VSi a of vanadium and Si VSi Two compounds silicon, V 2 and 2, have been prepared in the electric furnace. Vanadium. Subsilicide, V 2Si, is obtained by fusing a mixture of vanadium trioxide, V 2 3? and silicon, with the addition of either a large excess of vanadium or carbon or copper. The carbide or copper alloy 11 produced is decomposed at the temperature employed. The silicide forms metallic prisms, of density 5-48 at 17 C., the m.pt. of which is higher than in the case of the disilicide. It is attacked by the halogens, hydrogen chloride gas, and fused sodium or copper, but hydrochloric acid, nitric acid and sulphuric acid are without action- VSi is a mixture of Vanadium. Disilicide, a , prepared by reducing silicon vanadium trioxide and with magnesium ; vanadium pentoxide 12 can also be employed with the addition of fluorspar to the mixture. 13 Gin used a mixture of the pentoxide, silica, and coke. Vanadium is stable it forms metallic which disilicide a very compound ; prisms melt at 1655 C., have a specific gravity of 4-42, and are hard enough to scratch glass. It is attacked by hydrofluoric acid, hydrogen chloride gas, and fused alkalis. and vanadium V both Hypovanadic oxide, V0 2 , pentoxide, 2 5 , dissolve in hydrofluosilicic acid to yield a vanadyl fluosilicate and a 14 vanadium fluosilicate, both of doubtful composition. A compound of vanadium, aluminium, and silicon is referred to are on p. 28. The molybdovanadosilicates and tungstovanadosilicates described on pp. 88 and 89. VANADIUM AND BORON. Vanadium Boride, VB, is prepared by fusing a mixture of the elements in vacuo at the temperature of the electric furnace. It is a

1 224 ZeitecL 1927, Gain, Ann. Ghiin. Phys., 1908, fviii], 14, ; Schramm, anorg. Chem., L 161, 231 ; see also Berzelius, Poytj. Annalen, 1831, 22, 2 ZeitscJi. EhUro- Piecini and Brizzi, Zeitech. amrg. Chem., 1899, 19, 394 ; Biiltemann, chem., 1904, 10, 141. 3 Koppel and Goldmann, Zeitsch. anorg. Chem., 1903, 36, 381. 4 Rosenlieim and ZeitedL Barbieri, Atti R. Acead. Lincei, 1914, 23, 47, 408 ; Mong, anorg. Chem,., 1925, 148, 131. 5 Canneri, Gazzetta, 1926, 56, 637, 901. 6 See p. 90. 7 Barbieri, Atti &. Accad. Lincei, 1915, [v], 24, 724. 8 Scarliarini and Airoldi, Gazzetta, 1925, 55, 44. 9 Meyer and Markowitz, Zeitsch. anorg. Chem., 192(3, 157* 211. 10 Giebelhausen, ibid., 1915, 91, 251. 11 Moissan and Holt, Ann. Ohim. Ptys., 1902, [vii], 27, 277. 1 2 Meyer and Baoka, Zeitsch. anorg. Chem., 1924, 135, 177. is Gin, EUctrochem. Met. Ind., 1909, 7, 264. i* 169 210 ; Bull Soc. chtm., Berzelius, Pogg. Annalen, 1824, I, ; 1824, 2, Guyard,

1876, [ii], 25, 352. 108 VANADIUM, NIOBIUM, AND TANTALUM. hard, metallic substance, which is attacked by nitric acid, hydrofluoric 1 acid, and fused alkali. A vanadium borate of uncertain composition is obtained by fusing a mixture of vanadium pentoxide and boric anhydride. It forms green or yellowish-green crystals which, however, undergo hydrolysis 2 readily. A vanadyl borate is also obtained by the action of borax on 3 vanadyl sulphate solution, and a complex vanadium borotungstate has been prepared. 1 \Vcdekind, Bt>f., 1913, 46, 1198. 2 Ouyard, Bull Xor. chim., 1876, [ii], 25, 350. 3 Berzelius, Pogy. Annalen, 1831, 22, 1. CHAPTER IV. THE DETECTION AND ESTIMATION OF VANADIUM.

Detection. Apart from naturally occurring ores of vanadium, vanadium steels, and ferrovanadium, the commonest compounds of vanadium are those which contain the element in the peiitavalent state, viz. the pentoxide and the various vanadates. The analytical reactions usually employed are, therefore, those which apply to vanadates. Most vanadium ores can be prepared for the application of these reactions by digesting with mineral acids or by alkaline fusion with the addition of an oxidising agent. When the silica content is high, preliminary treatment with hydrofluoric acid is recommended. Vanadium steels and bronzes, and ferrovanadium, are decomposed by the methods used steels for other ; the drillings are, for instance, dissolved in sulphuric acid and any insoluble carbides then taken up in nitric acid, or they are filtered off and submitted to an alkaline fusion. Compounds of lower valency are readily converted into vanadates by oxidation with bromine water, sodium peroxide, or potassium permanganate. Wet Tests. (a) When rendered faintly acid, colourless solutions of vanadates become yellow, and, with the addition of more acid, orange- red, in consequence of the formation of polyvanadates. (b) Hydrogen sulphide, on being passed through an acidified solution of a vanadate, does not give rise to a precipitate, but a blue solution results in consequence of the formation of a vanadyl salt. Blue solutions are obtained also by the action of other reducing agents (seep. 57). This reaction is not peculiar to vanadium compounds, since salts of molybdenum also give blue solutions on being reduced. In the case of vanadium, however, by the use of zinc, cadmium or aluminium, reduction can be made to proceed still further, with the formation of a green, and finally a lavender, solution. solution of (c) Ammonium sulphide, on being added to a neutral a vanadate, gives rise to a brownish-red solution which contains a thiovanadate. Ammonium sulphide cannot, however, be employed to separate those metals normally precipitated by this reagent, since vanadates of zinc, manganese and nickel are thrown down when the solution is rendered alkaline. When the thiovanadate solution is acidi- the of fied, partial precipitation of the vanadium takes place in form sulphides or oxysulphides. the alkali (d) Ammonium chloride solution has no action on vanadates, but if sufficient of the solid is added to form a saturated solution, colourless ammonium metavanadate is precipitated. action on acid solu- (e) Ammonium hydroxide is similarly without tions of alkali vanadates, but if added to a vanadate solution which 109 110 VANADIUM, OTOBIUM, AND TAJSTTALUM.

contains cations of ferric iron, aluminium, chromium, uranium, barium, In the etc., vanadates of these metals may be precipitated* ordinary salt which is in the process of group analysis, the vanadyl present reoxidation filtrate from the hydrogen sulphide group may undergo to the vanadate when this filtrate is warmed with nitric acid previous the to adding ammonium hydroxide for the precipitation of hydroxides can be carried of iron, aluminium, and chromium. This oxidation solution. For the to completion by boiling with hydrogen peroxide iron and the separation of vanadium from aluminium, precipitate is redissolved in first produced on addition of ammonium hydroxide or three times. If nitric acid, and the precipitation is repeated two the vanadium the precipitate is boiled with ammonium phosphate, the iron and aluminium in the residue. passes into solution and leaves the addition of excess Iron can also be separated from vanadium by of of sodium carbonate and sodium peroxide to an acidified solution out as ferric and the salts, whereupon the iron is thrown hydroxide the vanadium remains in the filtrate as vanadate. Traces of vanadium the latter in occluded in the precipitate can be removed by dissolving the minimum quantity of nitric acid and repeating the precipitation. An alternative method for the separation of aluminium from vanadium of sodium aluminate depends on the fact that a boiling dilute solution when and sodium vanadate precipitates aluminium hydroxide it^is treated with a large excess of ammonium nitrate in small quantities at a time; addition of barium chloride to the filtrate precipitates 1 barium metavanadate. If chromium is present it will have been oxidised to chromate by the hydrogen peroxide or sodium peroxide treatment, and will be left in solution with the vanadate after removal of iron and aluminium. In order to separate the chromium and vanadium, the acidified chromate-vanadate solution is reduced with the sulphur dioxide, wanned with bromine water (which oxidises vanadyl salt back to the vanadate without affecting the chromium salt), and then poured into a 10 per cent, solution of caustic soda. Chromium 2 hydroxide is precipitated, while the vanadate remains in solution. An alternative method for the separation of chromium and vanadium acetic acid and consists in precipitating with lead acetate ; hydrogen the peroxide are added to the vanadate-chromate solution, whereby

chromate is reduced to the chromic salt ; addition of lead acetate at this stage throws down lead vanadate, the chromium salt remaining in solution. (/) The colour change produced by the addition of hydrogen peroxide to a strongly acid solution of a vanadate constitutes the commonest qualitative test for the presence of vanadium. High concentrations of vanadium give rise to an intense reddish-brown coloration (see tint. p. 91), while very low concentrations give a faint rose-red The 3 reaction is sufficiently delicate to detect 1 part of vanadium in 160,000. In applying the test it is necessary to remember that chromium, titanium and iron also produce comparable colorations. In the case of chromium, however, the further addition of ether gives a blue colour, whereas the to is 4 acid colour due vanadium unaffected ; addition of hydrofluoric

1 Wenger and Vogclson, Reli\ Chim. Ada, 1919, 2, 550. 3 J. Browning, Amer. Chem. 8oc. 9 1921, 43, 114. 3 Meyer and Pawletta, Zeitsch. anal Ghem., 1926, 69, 15. * Reichard, ibid., 1901, 40, 377. THE DETECTION AND ESTIMATION OF VANADIUM. Ill or of ammonium fluoride destroys the colour due to titanium, and any yellow colour due to the presence of iron is removed by addition of phosphoric acid. (g) Tannin yields a rich blue coloration with vanadates in concentrations down to 2 mgm. of vanadium pentoxide per litre. Gallic acid and pyrogallol also give a blue colour. Commercial ethers, which contain vinyl alcohol, give rise to a rose coloration in concentrations down to 0*1 mgm. of vanadium pentoxide per litre, or even as low as 0-02 litre if the of alcohol is increased.1 " mgm. "per proportion vinyl Cupferron produces a red coloration with weak solutions of vanadium 6 2 salts even when the dilution is 10~ mgm. per c.c. A 0-2 per cent, aqueous solution of diphenylamine in the presence of hydrochloric acid gives a violet coloration with aqueous solutions of vanadium com- this colour is not affected pounds ; by the presence of nitrates, titanates, or iron, and detects vanadium in a solution which contains 0*0002 per 3 cent. Addition of an extremely dilute solution of potassium thio- cyanatc and a trace of sulphuric acid gives a yellow coloration which becomes blue with further addition of sulphuric acid ; this reaction is sensitive to 1 part of vanadium pentoxide in 5000. 4 Vanadates also 5 6 7 give colour changes with resorcinol, quinine, morphine, strychnine, 8 9 phenol, aniline, etc., and have, therefore, been employed from time 10 to time as analytical reagents for these organic compounds. Solutions of vanadates which are (/?,) neutral or faintly acid with acetic acid readily yield precipitates of vanadates of the heavy metals. Silver nitrate, with a carefully neutralised solution, produces a curdy, reddish-brown precipitate, soluble both in ammonium hydroxide and in nitric acid. Mercurous nitrate throws down a yellow precipitate of mercurous vanadate, which is soluble in nitric acid. Lead acetate gives a yellow precipitate, which dissolves in nitric acid and becomes white on standing. Orthovanadates can be distinguished from metavanadates by the colours of the copper salts which they throw down on the addition of metavanadates a copper sulphate ; yield yellow, crystalline precipitate, 11 while orthovanadates yield an apple-green precipitate. These colours vary, however, with the acidity of the solution. (i) Solutions of pyrocatechol acetate and aniline, or pyrocatechol 12 acetate and piperazine, are sensitive micro-reagents for vanadium. Dry Tests. (a) When strongly heated in a borax bead vanadium compounds impart a yellow coloration to the oxidising flame and a light green coloration to the reducing flame, (b) When heated in a bead of microcosmic salt in the oxidising flame, vanadium compounds impart a brownish-red coloration to the bead, which becomes orange a colour is on cooling ; in the reducing flame brownish-green produced, (c) Vanadium compounds do not colour the ordinary flame. 1 Matignon, Compt. rend., 1904, 138, 82. 2 Rodcja, Anal Fis. Quim., 1914, 12, 305. 3 Meaurio, Anal. Soc. Quim. Argentina, 1917, 5, 185. * Ellram, Ghem. Zentr., 1896, [ii], 211. 5 Levy, Compt. rend., 1886, 103, 1195. 6 von'Klecki, Zeitscti. anorg. Chem., 1894, 5, 374. 7 Reichard, Zeitsch. anal Chem., 1903, 42, 95, 293. 8 Truchot, Ann. Chim. anal, 1902, 7, 167. 9 Laar, Ber., 1882, 15, 2080. 10 Mandelm, Zeitsch. anal Chem., 1884, 23, 235 ; Dragendorff and Johannson, Jahresber., loc. cit. 1884, 1646 ; Keiohard, 11 12 Roscoe, J, Chem. Soc., 1871, 24, 31. Martini, British Chem. Abs. 9 A, 1926, 1244. 112 YANAJ3ITJM, NIOBIUM, AND TANTALUM.

Estimation of Vanadium*1 Volumetric Methods. The most convenient and the usual method for the estimation of vanadium is a first obtained in acid solution volumetric process. The vanadium is several as vanadate, and reduced to the tetravalent state by one of is then titrated in the reducing agents which are available. The solution solution, presence of sulphuric acid with hot potassium permanganate salt to the vanadate. which quantitatively oxidises the lower vanadium the reactions Using sulphur dioxide to effect the reduction, following take place : =2VOS0 0. (i) 2HV0 3 +SO a +H 2S0 4 4 +2H 2

0=10HV0 S0 4 +2MnS0 4 (ii) !OVOS0 4 +i>KMn0 4 +12H2 3 +K 2

the titration Excess of sulphur dioxide is removed before by boiling 2 the reduced solution in an atmosphere of carbon dioxide. Hydrogen of dioxide and sulphide can be used in place sulphur gives slightly excess is removed in the same manner boiling, and higher results ; by before 3 the precipitated sulphur is removed titrating. Hydrogen peroxide, when added in small proportions to a concentrated sulphuric acid solution of a pentavalent vanadium salt, also brings about reduction of the is decom- to the tetravalent state ; excess hydrogen peroxide formed.4 Other posed catalytically by the vanadyl sulphate reducing are bismuth 5 6 agents which have been employed amalgam, mercury, 7 8 sodium thiosulphate, concentrated hydrochloric acid, and electrolytic methods.9 The use of hydrochloric acid has not always given good results in the hands of different chemists, since reduction has a tendency tetravalent 10 the use of alcohol and to proceed lower than the stage ; 11 hydrochloric acid has been recommended. The estimation of a vanadate solution by direct titration with ferrous solution has been worked sulphate or ferrous ammonium sulphate out, and is found to be specially applicable to the analysis of vanadium to the tetravalent state the alloys. The vanadate is again reduced by ferrous salt, the end point being obtained by the use of potassium excess of ferricyanide as internal indicator; alternatively, a known the ferrous salt solution is added to the vanadate, the amount unused

1 The bibliography on the estimation of vanadium up to the year 1901 is given by Brearley, Ghent. 2Tew*, 1901, 83, 163. * Gerland, Ber., 1877, 10, 1513, 1516; Holverseheit, Dissertation (Berlin, 1890); Hffle- J. Ird. brand, /. Amer. Ohem. Soc., 1898, 20, 461 ; Cain, Eng. Chem., 1911, 3, 476. 3 Lundell and J. Atner. Ohem. Koenig, Stahl mid Eisen, 1914, 34, 405 ; Knowles, 8oc., 1921, 43, 1560. * J. Gain and Hostetter, ibid., 1912, 34, 274 ; Hothersall, Soc. Ghem. Ind., 1924, 43, 270. fi Someya, Zeitsch. anorg. Ghent,., 1926, 152, 368. * McCay and Anderson, J. Aiwr. Ghem. Soc., 1922, 44, 1018. 7 Oberhelman, Aimr. J. ScL, 1915, pv], 39, 530. 8 Gooch and Stookey, ibid., 1902, [iv], 14, 369; Campagne, Ber., 1903, 36, 3164; Cain, J. Ind. Eng. Ohem., 1911, 3, 476. 9 Bleecker, Met. Ghem. Eng., 1911, 9, 211 ; Gooch and Scott, Amer. J. Sci., 1918, [iv], 46, 427. J0 Compare Rosenheim, Annalen, 1889, 251, 197; Holverseheit, loc. cit. Auchy, J. Ind, Eng. Ghem., 1909, i, 455 ; Wilms and Fischbach, Stahl und Eisen, 1914, 34, 417 ; Hartmann, Zeitsch. anal Ghem., 1925, 66, 16. 11 Zeitsch. Midler and Diefenthaler, anorg. Chew., 1911, 71, 243 ; Wegelin, Zeitech. anal Ohem., 1914, 53* 81. THE DETECTION AISTD ESTIMATION OJT VANADIUM. 113 being titrated with potassium diehromate.1 This method facilitates the rapid estimation of vanadium in the presence of chromium. Knop's indicator (dipheiiylamine in sulphuric acid) has recently been success- 2 fully employed. Vanadium lends itself also to iodometric methods of determination. The vanadate solution is reduced with liydrobromic acid, excess of potassium iodide is added, and the iodine thereupon liberated is esti- 3 mated with sodium thiosulphate solution. The reaction is :

2HVO 8 +2HBr+4HCl=2VOCl a +Br 2 +4H aO.

This method is also available for the estimation of vanadium and 4 chromium together in solution. The use of hydriodic acid as the reducing agent gives rise to inconsistent results, as reduction does not stop at the tetravalent stage, but may proceed also to the trivalent stage, 5 according to the experimental conditions. The care necessary in the application of iodometric methods renders it unlikely that they will 6 come into general use. Treadwell 7 has recently shown that vanadium in acid solution is reduced quantitatively to the divalent state by electrolytically deposited cadmium, zinc amalgam, or lead amalgam, if air is carefully solution is run into excess of excluded ; the reduced potassium permanganate and titrated with oxalic acid, or it may be oxidised to the tetravalent state by the addition of excess of copper sulphate solution and then titrated with potassium diehromate, using dipheiiylamine as indicator. A modification of this procedure consists in running the acidified vanadate solution through a Jones reductor which contains of ferric alum solution the amalgamated zinc into an excess ; quantity of ferric salt thereby reduced is determined by back titration with potassium permanganate. 8 Other volumetric processes which have been worked out include 9 10 the use of potassium ferrocyanide, potassium ferricyanide, titanous 12 13 chloride,11 and stannous chloride. According to Rosenheim and Yang, vanadium pentoxide is best determined in solution by addition of excess of caustic soda and back titration with sulphuric acid at 100 C., using a-naphthophthalein as indicator. involves the The application of any of the foregoing processes previous

1 389 Met. Chem. 1913, II, 195 ; Williams, J. Soc. Chem. Ind. t 1902, 21, ; Clark, Eng., 588. Etheridge, Analyst, 1923, 48, , H ^ T T , rt^ 2 237 J, Soc. Chem. Ind., 1926, Someya, Zeitsch. anorg. Ohem., 1924, 139, ; Russell, MJ- ' gijr. 3 (iooch and Curtis, Amer. J. Sci., 1904, [iv], 17* 41. * 333. Edgar, ibid., 1908, [iv], 26, . cit. J. s Gooeh and Curtis, loc. ; Amer. Ditte, Compt. rend., 1886, 102, 1310 ; Edgar, z and Zeitsch. anal Chem , 1914, 53< 80 ; D& Bardach, Chem. Soc., 1916, 38, 2369 ; Wegelin, Sidener, and Brinton, J. Amer. Chem. &oc., Zeitsch. anorg. Chem., 1915, 93, 97 ; Stoppel, Zeitsch. anal 1926, 68, 461. 1924, 46, 2448 ; Heezko, Chem., 6 J. Amer. Chem. Soc., 1927, 49, 1138. Compare Heczko, loc. cit. ; Ramsey, * 563 ; 1922, 5, 739. Treadwell, Helv. Chim. Acta, 1921, 4, . * 761 see also Zeitsch. anorg. Nakazono, J. Chem. Soc. Japan, 1921, 42, ; Someya, Chem., 1927, 163, 206. 9 Muller and Diefenthaler, Zeitsch. anal Chem., 1912, 51, 21. 10 Palmer, Amer. J. Sci., 1910, [iv], 30, 141. 11 Atack, Analyst, 1913, 38, 99. 12 WarinsM and Mdivani, BuU. Soc. chim., 1908, [iv], 3, 626. 13 Rosenheim and Yang, Zeitsch. anorg. Chem., 1923, 129, 181. S YOL. VI. : III. 114 VANADIUM, NIOBIUM, AKD TANTALUM. of other elements which may Interfere. For details of these separation 1 the reader is referred to standard works on analysis. of Electrometric Methods have been applied for the estimation vanadium alone and alloyed with other metals, e.g. iron, chromium, uranium. The reduced solution is either gradually oxidised by means ammonium of a suitable oxidising agent (potassium permanganate, solution is reduced persulphate, nitric acid), or the vanadate gradually solution the in the E.M.F. of a suitable with ferrous sulphate ; changes 2 cell indicate the end point. small Colorimetric Methods are used only for the estimation of very in vanadium steels and The percentages of vanadium, e.g. alloys. of the reddish-brown colour most important depend on the intensity on an acid vanadate produced by the action of hydrogen peroxide 3 must be intro- solution. If chromium is present, an equal amount duced into the standard vanadium solution under the same conditions acid is added to of temperature, acid concentration, etc. Phosphoric colour due to ferric iron, and either hydrofluoric destroy any yellow 4 acid or ammonium fluoride to destroy any colour produced by titanium. A colorimetric method for the simultaneous estimation of small quantities 5 of titanium and vanadium has also been worked out. Other colori- the formation of a to black metric processes are based on (a) yellow coloration, due to aniline black, in the presence of aniline hydrochloride 6 and the and potassium chlorate or other oxidising agent, (b) orange acid solution of a vanadate is coloration finally produced when an 7 brought into contact with strychnine sulphate. 8 has an Electrolytic Method. Truchot developed electrolytic process as for the estimation of small quantities of vanadium in solution is rendered alkaline with ammonium vanadate ; the solution faintly hydroxide, and on passing the electric current a lower oxide is deposited, which is collected, converted to the pentoxide by heating in air, and weighed. Arc Spectrum. Vanadium has been estimated in ores with fair accuracy by comparative measurement of the intensity of the lines in 9 the arc spectrum. Gravimetric Methods.- The vanadium compound is converted into sodium vanadate by fusion or other method, and after separation from other salts (e.g. arsenate, molybdate, phosphate, chromate, tungstate) is precipitated from nearly neutral solution either as (a) mercurous vanadate or (b) basic lead vanadate. In (a), mercurous 1 See also, for a particularly good account, Analytical Methods for Certain Metals, Bulletin 212, 1923, Bureau of Mines, Washington, 2 and Kelley and Conant, J. Amer. Chem. Soc., 1916, 38, 341 ; Kelley, Wiley, Bohn, J. Chem. Wright, /. 2nd. Eng. Chem., 1921, 13, 939 ; Gustavson and Knudson, Amer. Soc. Mtiller and Zeitsch. 155 Muller and t 1922, 44, 2756 ; Just, anorg. Chem., 1922, 125, ; Flath, Zeitsch. EUUrochem.. 1923, 29, 500; Willard and Fenwick, J. Amer. Chem. Soc., Trav. 1923, 45, 84 ; Kolthoff and Tomicek, Eec. chim., 1924, 43, 447, 3 Meyer and Pawletta, Zeitsch. anal. Chem., 1926, 69, 15. * Slavik, Chem. Zeit., 1910, 34, 648 ; Pickard, Chem. World, 1914, 2, 341 ; Fenner, Chem. Zeit, 1918, 42, 403; McCabe, Chem. News, 1911, 104, 194; J. Ind. Eng. Chem.,

1913, 5, 736 ; Georges, Bull de Bety., 1922, 31, 123 ; Kropf, Zeitsch. angew. Chem. 9 1922, 35, 366 ; Meyer and Pawletta, Zeitsch. anal Chem., 1926, 69, 15. 5 Mellor, Trans. Eng. Geram. Soc., 1913, [i], 12, 33. 6 Soc. J. Witz and Osmond, Bull chim., 1886, [ii], 45, 309 ; McAdam, Amer. Chem. Soc., 1910,32,-1611. 7 8 Gregory, Chem. News, 1909, 100, 221. Truchot, Ann. Chim. anal, 1902, 7, 165. 9 and Porlezza Donati, ibid., 1926, 16, 519 ; 1927, 17, 3. THE DETECTION AND ESTIMATION OF VANADIUM. 115 nitrate solution is added to the vanadate solution drop by drop until no further precipitation takes place ; the mercurous vanadate so obtained is heated under a the is hood, whereupon mercury volatilised ; the residue of is pure vanadium pentoxide weighed ; hydrochloric acid 1 2 should not be present. The results have a tendency to be high. In (b), lead acetate solution is added to a solution of the vanadate rendered faintly acid with acetic acid, whereupon all the vanadium is precipitated as a basic lead vanadate of variable composition. The precipitate is dissolved in nitric acid, the lead removed by boiling with sulphuric acid, and the filtrate, which contains vanadic acid, is then either to the residue as or evaporated dryness and weighed V 2O 5 , the vanadic 3 acid in it is estimated by a volumetric process. For the"application of either of these methods, the removal of arsenic is effected by reducing solution of arsenate the acidified vanadate and with sulphur dioxide ; the arsenic is then precipitated as sulphide with hydrogen sulphide, and the vanadium which remains in solution as the vanadyl salt is reoxidised to the vanadate for estimation. Molybdenum is separated by a similar process, except that the hydrogen sulphide is used under 4 pressure. For the estimation of vanadium in the presence of a phosphate, mercury vanadate and phosphate are precipitated together, ignited, and the residue of vanadium pentoxide, after being weighed, converted into sodium vanadate and phosphate by fusion with sodium carbonate. The vanadate is converted into the vanadyl salt by reduction with sulphur dioxide and the phospHate determined by means of ammonium molybdate. -Deduction of the equivalent quantity of phosphorus pentoxide from the weight of mixed oxides gives the vanadium content. A general method for the separation of vanadium from arsenic, molybdenum, phosphorus, chromium, uranium, tungsten, and silicon, consists in precipitating these metals as their respective lead salts and digesting the precipitate with potassium carbonate, whereupon all the 5 lead salts are decomposed with the exception of the lead vanadate. The gravimetric estimation of vanadium in alkaline vanadate solutions has also been effected by precipitating as ammonium meta- 6 vanadate in the presence of ammonium chloride. Precipitation is incomplete, however, unless the solution is quite saturated with 7 is 8 ammonium chloride ; the addition of alcohol recommended. Other gravimetric processes which have been investigated include the 9 precipitation of barium pyrovanadate, precipitation of silver meta- 10 11 vanadate, precipitation of manganese pyrovanadate, and the use of 12 cupferron.

1 Auerbach and Lange, Zeitsch. angew. Chem., 1912, 25, 2522. 2 Bleecker, Met. Chem. Eng., 1911, 9, 213. 3 Ann. Ghtm. Roscoe, Annakn, Suppl., 1872, 8, 102 ; Cormimbceuf, anal, 1902, 7, 258. * 353. Stoppel, Sidener, and Brinton, Ohem. News, 1925, 130, 5 Fischer, Dissertation (Kostock, 1894). 6 982 von Hauer, J. praJct. Chem., 1856, 69, 388 ; Ditte, Compt. rend., 1887, 104, ; Rosenheim, Zeitsch. anorg. Chem., 1902, 32, 181. 7 Gooch and Gilbert, Amer. J. Sri., 1902, [iv], 14, 205. 9 8 Beard, Ann. Chim. anal, 1905, 10, 41. Carnot, Compt. rend., 1887, 104, 1803. 10 Browning and Palmer, Amer. J. Sci., 1910, [iv], 30, 220. 11 Carnot, CompL rend., 1887, 104, 1844. * 2 /. Turner, Amer. /. Sci., 1916, 42, 109 ; Rolla and Nuti, Chem. Soc., Abs., 1921, 466. 20, pi], 597 ; Clarke, Analyst+LVZl', 52, 116 VANADIUM, NIOBIUM, AND TANTALUM.

The analysis of vanadium steels is effected by the application of one of the foregoing methods. Blank determinations on a steel free from vanadium but otherwise of the same approximate composition are used as a control. Iron and molybdenum are removed from hydrochloric acid solution ether 1 by Rothe's separation method ; chromium, nickel, copper, etc., are then precipitated as hydroxides by caustic 2 soda, the filtrate containing- the vanadium as vanadate. The method is modified for the simultaneous estimation of both vanadium and chromium in a vanadium-chromium steel.3

1 Ptothe, ZeitscL ami CJicm., 1901, 40, 809; Deiss and Leysaht, Chem. Zcit., 1911, 35, 869, 878. 2 J. J. JBIair, Awer. Cfam. 8oc., 1908, 30, 1229 ; compare Dougherty, Ind. Eng. Ckcm.,

1915, 7, 419 ; Demurest, ibid., 1912, 4, 249. 3 Bu.ll. 6'or. 9<>2 J. Campagne, chitn., 1904, 31, ; Edgar, Amer. 8cL, 1908, [iv], 26, J. 333 ; Palmer, ibid., 1910, [iv], 30, 141 ; Cain, Ind. Etuj. Ckem., 1911, 3, 476. CHAPTER V.

NIOBIUM 1 AND TANTALUM.

THEIR OCCURRENCE, HISTORY, EXTRACTION, ESTIMATION, AND DETECTION.

Occurrence. Niobium is almost always associated with tantalum in its natural ores, so that it will be convenient to consider the occurrence of both these elements together. There are very few niobium-bearing minerals which are free from tantalum and vice versa. Niobium and tantalum do not occur naturally in the free state to extent native tantalum small amounts of nickel has, any ; containing however, been found in the gold washings from the Ural and Altai Mountains.2 The metals are found mainly as negative radicals in minerals in which the oxides of iron, manganese, calcium, and various rare earth metals, for example yttrium, thorium, lanthanum, cerium and uranium, form the bases. Titanium, zirconium and tin, as well as other rare earth metals, are also frequently present. The minerals are small numerous, and are very generally distributed in quantities over the earth niobium in apparently greater quantity than tantalum. in North The largest deposits have been found America, Greenland, Finland, Sweden, the Ural Mountains, Bavaria, and Australia. and tables The most important ores are described below, showing 3 typical analyses are appended. Tantalites. These are ferrous Niobites (or Columbites) and mainly salts of metaniobic acid and metatantalic acid in which the iron is more can be formulated or less replaced by manganese. They generally i.e. the salts are niobates and , present Fe(Mn)(NbO 3 ) 2 Fe(Mn)(TaO 3 ) 2 If the niobium is in excess and tantaktes (see pp. 160 and 200). they if the tantalum is in excess are called niobites (or columbites), and they are called tantalites. There is no definite line of demarcation between the iron salts. Tin and are two classes. Tapiolite contains only tungsten and some of niobites also frequently present in small amounts, samples two of man- have recently been found to contain the higher homologues has been 1 Niobium is also known as columbium. The latter name always preferred In columbium in America, and is retained, no doubt, out of patriotism. England the^name 1923 inclusive ; in 1924 the name was employed by the Chemical Society from 1904 to the element has been more niobiuni was readopted. On the Continent consistently referred to by the various translations of niobium. 2 350 ; Walther, A ature, Dana, Text Book of Mineralogy, 3rd ed. (New York, 1850), p. 398. 1909, 81, 335 ; von John, ibid., 1910, 83, * 883. All analyses of minerals containing Schilling, Zeitsch. angew. Chem., 1905, 18, The densities, tantalum previously published are here collected by Schilling. percentages in which the minerals occur, and the of niobium and tantalum present, the localities references to the literature, are tabulated. 117 118 VANADIUM, NIOBIIB1, AND TAOTALUM. ANALYSES OF NIOBITES OR COLUMBITES.

tantalites are ganese, masurium and rhenium.* Niobites and usually found in igneous rocks, and are not uncommon as constituents of cassiterite-bearing pegmatite veins. Their chief localities are the Black Hills of South Dakota, U.S.A., Greenbushes and Wodgina in Western Australia, and on the Finniss River near Port Darwin in Northern Australia, from all of which regions tantalites for industrial use have been obtained, although, in consequence of the restricted demand, the total output has been small and irregular. Single masses of niobite weighing up to 2000 Ib* have been discovered in the granite veins in the Black Hills of South Dakota, while at Wodgina the tantalite occurs in crystalline masses weighing up to about 550 Ib. It is collected from the surface soil of the neighbouring alluvial deposits or by quarrying the pegmatite. Most native niobites and tantalites offer considerable resistance to chemical change, and as they are both hard and tough, they occur frequently in detrital deposits. These are, however, usually 6 overlooked unless they happen to be worked for gold or tin. Other places where ores are known to occur are Greenland, Bavaria, Finland, Miask in the Ural Mountains, Chanteloube near Limoges in France, California, and Colorado. For recent observations of their occurrence in the British Empire see the references cited. 7 1 Brogger, /. Ch&m. Soc., Abs. t 1907, ii, 92, 885. 2 Blomstrand, J. prakt. Ckem., 1866, 99, 44. 3 Headden, Atmr. J. Sci., 1922, [v], 3, 296 ; see also J. Chem. Soc., Abs., 1906, [ii], 90, 37. 4 The absence of titanium from these ores is remarkable. 5 Nbddack and Tacke, Oesterr. Ghem- Zeit., 1925, 28, 127 ; Noddack and Noddack,

fl Zeitscli. physikal Chew., 1927, 125, 264. Simpson, Chem. News, 1909, 99, 49. 7 Bhodesia: Zealley, Nature, 1918, 101, 174. South Africa: Bogers, Trans. Geol. Soc. 3. Africa, 1916, 18, 5 ; Reuning, Zeitsch. Kryst. Min,.. 1923, 58, 455. India : Tipper, Rec. Oeol. Survey India, 1919, 50, 255. Canada: Ellsworth, Can. Deyt. Mines, Summary Report, 1923, C, I, 6. Australia : Simpson, loc. cit. NIOBIUM AND TANTALUM. 119 ANALYSES OF TANTALITES.

The natural niobites and tantalites are usually black, and form isomorphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8*2 with increase in tantalum content. When heated to redness in vacuo they evolve small quantities of gas, 7 which consists of carbon dioxide, nitrogen and oxygen. Small quantities of helium have also been found occluded in them. also contain Pyrochlore. This is a crude calcium niobate which may with appreciable quantities of titanium, thorium and cerium, together smaller quantities of iron, magnesium, the alkali metals, and fluorine. It does not contain chlorine, and it is of interest In that some specimens are remarkably free from tantalum. It occurs in Norway and near Miask in the Ural Mountains. The ore is brown, forms regular octa-

1 at in the Pilbara Simpson, Chem. News, 1909, 99, 77. This ore occurs Wodgina Goldfield in the north of Western Australia, and is shipped to Europe in the quantities required for the extraction of its tantalum. * Chem. Weiss and Landecker, Zeitsch. anorg. Che?n., 1909, 64, 65 ; News, 1910,

3 65 Chem. 1871, 23, 60, 71, 28G ; Eammelsberg, Pogg. Annakn, 1871, 144, ; News,

4 see also Shibata and Kimura, J. Chem. 'Headden, Amer. J, SGI., 1922, [v], 3, 296 ; J. Soc. 122, ii, 220. Soc. Japan, 1921, 42, 957 ; Chem. f Ab$., 1922, 5 The absence of titanium from this ore is remarkable. and Univ. Toronto Studies, Geol Miigge, Centr. Min., 1924, 417 ; Walker Parsons, Headden, Amer. J. Set. 1922, Ser. 9 1923, No. 16; Zeitsch. Kryst. Min., 1925, 61, 353; Zeitsch. 1909, 65 ; Chem. A'ews, (YL 3, 393 ; Weiss and Landecker, anorg. Chem., 64, J. 393 WeinLmd and 1910, loi, 28; Poote and Langley, Amer. Sci., 1910, [iv.], 30, ;^ /be. cit. Hall and Smith, Proc. Amer. Herz, Zeitsch. anorg. Chem., 1907, 54, 230 ; Brogger, ; 220 Hall, /. Amer. Chem. Soc., 1904, Phil Soc., 1905, 44, 177 ; Chem. News, 1905, 92, ; 684 J. Chem. 26, 1235; Tschernik, J. Zuss. Phys. Chem. Soc., 1902, 34> ; SoÂîm, and Hall, London), 1899, 6th ed., ii, 84, 157 ; Dana, A System of Mineralogy (Chapman pp. 731 et $eq. 7 Chabrie^ and Levallois, Compt. rend., 1906, 143, 680. 120 VANADIUM, NIOBIUM, AND TANTALUM, fracture. Its varies hedra, is brittle, and presents a conchoidal density from 4-2 to 4-5. * 7 These., oip Niobales and Tantalales of the Rare Earth Metal*. âre rather than for their niobium importance for their rare earth content and tantalum. Examples are : other of Sweden, Yttrolantalite, which is found at Ytterby and parts and contains and in Norway. It is richer in tantalum than in niobium, and uranium, to- considerable proportions of yttrium, erbium, cerium, 2 are also with calcium, iron, etc. Tungstates and stannates present. gether and m Fergusonite.Tbis ore is also found at Ytterby (Sweden), and Its com- Norway, Greenland, Texas, South Africa Ceylon. that the niobium position is comparable to that of yttrotantahte, except lanthanum is content is than that of the tantalum, and usually greater 3 also found in the basic portion. According to Rammelsberg, fergusonite meta- from Greenland consists of isomorphous mixtures of yttrium . It and metatantalate, Y 2 3.Ta 2 5 niobate, \%O a.Nbo0 5 , yttrium of which varies forms brown or black tetragonal crystals, the density 4 evolution from 4-3 to 5-S. It is radioactive, and glows suddenly with of helium when heated from 500 to 600 C. SamarsJdte occurs in the Ural Mountains, Mitchell County (North content is often Carolina, U.S.A.), Canada, and India. The tantalum and ceria, small, sometimes nil, and the rare earth oxides, chiefly yttria The ore are usually present in considerable number and proportions. is radioactive and contains helium. It forms black, orthorhombic 4-2 to G-2.5 It has been crystals. The density varies from suggested that the niobium and tantalum are disintegration products of compounds of 6 of yttrium and cerium with the two higher homologues manganese, masurium, and rhenium. and differ Euxeniie, ceschynite? and potycrase are found in Norway, in composition from samarskite in that they usually contain considerable 8 quantities of titanium. Tantalum is not always present. Wolderite is a niobate of calcium, iron, manganese, sodium, etc., associated with considerable quantities of zirconia and silica. It is found in Norway. 9 Other silicates which contain niobium or tantalum are struverite io and ilmenorutile.11 of Tin and tungsten minerals frequently contain small proportions

1 naif 191 Jalitb. Rammelsberg, Pogg. An n, 1ST I, 144, ; 1873, 150,^198; Weidmann, J. HUM. Mitut,, 1909, [i], 22

94, 398 ; 1907, ii, 92, 3C4. 11 Zambonini and loc. cit. Jahrb. loc. cit. Prior, ; Miner., 1909, [i], 175 ; Brogger, NIOBIUM AND TANTALUM. 121 niobium and which are also tantalum, occasionally associated naturally with cryolite and pitchblende. ANALYSES OF PYROGHLORE, YTTROTANTALITE, FERGUSONITE, AND SAMARSKITE.

1 1,144, 191; 873, 150,! 98 ; 2 J. Russ. Ckem. Soc., J. Chem. Tscliernik, Phys. 1904, 36, 712; Soc., Abs. t 1904, ii, 86, 620 ; see also Tschernik, ibid.> 1909, ii, 96, 41L 3 see also cit. Brogger, ibid., 1907, ii, 92, 884 ; Rammelsberg, Zoc. 4 loc. cit. see also Zbc. cit. Brogger, ; Kammelsberg, ; Tscliernik, Zeitsch. Kryst, Min.> 625 SMbata and J. Ohem. 1904, 39, ; Kimura, Soc., Ab$., 1921, ii, 120, 269 ; Sato,

ibid., 1926, A, 934 ; Kimura, ibid., 1926? A, 144, 6 559 see also foe* Lacroix, Gompt rend., 1911, 152, ; Brogger, cit. ; J. L. Smith, J. 360 Chem. 29 Amer. Sci., 1877, 13, ; News, 1883, 48, 13, ; 1885, 51, 289, 304; Donald, ibid. J. 1 , 1 884, 49, 259 ; Roscoe, Ghem. Soc., 882,41,277 ; Gibbs, Amer. Ghem. J., 1893, 15, 546. 122 VANADIUM, NIOBIUM, AND TANTALUM. connected with History. The discovery of niobium is intimately are associated that of tantalum, firstly because these metals consistently because their together in their natural ores, and secondly, separation from one another has proved an extremely difficult matter. Indeed, considerable the chemistry of these elements is so closely parallel that were established. time elapsed before their separate identities definitely In 1801 Hatchett, 1 an English chemist, while working on some chromium minerals in the British Museum, examined a black mineral which had been found in the Connecticut valley and had been sent Hatchett the to the then president of the Royal Society. reported existence in the mineral of a new element, for which he suggested the name columbium, because the ore had come from America. The mineral itself became known as columbite. In the following year 2 in two other Ekeberg found what he thought was another new element and minerals, what is now known as tantalite, from Kimito (Finland), This new element was named yttrotantalite, from Ytterby (Sweden). were then common for tantalum, partly because mythological names that was new elements, and partly because of the tantalising difficulty the metal in excess of acids. experienced in dissolving the oxide of new 3 of two Wollaston subsequently compared the properties the new elements by re-examining their ores, and endeavoured to show that - then looked they were identical. It is now known that what was upon as the oxide must have been tantalum mixed with pure pentoxide" " the element was small proportions of niobium pentoxide, and new therefore impure tantalum. 4 In 1839 Wohler investigated some peculiar properties of an acid oxide (now known to be a mixture of niobic acid and titanic acid) Bavarian tantalites. present in the mineral pyrochlore and in some 5 columbites and tantalites Rose followed up the observation that many 9 as well as the acids obtained from them, displayed widely varying densities, and, after close investigation into their composition, he announced in 1844 that the columbites from Bodenmais in Bavaria contained, in addition to tantalic acid, the acid oxide of a metal which was not present in columbites from Sweden and Finland. The new metal was named niobium after Niobe, a daughter of Tantalus. In 1846 Rose thought that he had discovered the acid oxide of still another 6 metal, to which he gave the name pelopium, but later he decided that 7 this new acid was merely hyponiobic acid. In 1856-57 Hermann showed that both tantalum and niobium occur in the various natural tantalites and columbites. From a consideration of the composition of the halides, and because of the supposed isomorphism of tantalic acid and stannic acid, Rose gave tantalic acid the formulas NbO 2 and TaO 2 to anhydrous niobic acid and error been respectively, but in so doing he repeated an that had previously made by Berzelius with regard to vanadium compounds, and overlooked 8 the presence in the halides of an oxygen atom (see p. 24). Blomstrand

1 Hatchett, Proc. Eoy. Soc. f 1802, 92, 49. 2 K. Fdrh. Ann. 76. Ekeberg, Ofvers. VeL-Akad. t 1802, 23, 180; CJiim., 1802, 43, 3 Wollaston, Phil Tians., 1809, 99, 246; see also Berzelius, Pogg. Annahn, 1825, 4, 6; Cfvers. K. Vet.-Akad. ForL, 1824, z\ Ann. Chim. Phys., 1825, [iij 29, 300. * WoHer, Pogg. Annalen, 1839, 48, 91. s Kose, ibid., 1844, 63, 307, 693. 8 7 Bose, ibid., 1846, 69, 118. Rose, #&, 1853, 90, 471. 8 Blomstrand, /. prafa Gkem., 1866, 97, 37. NIOBIUM AND TANTALUM. 123

x and Marignac subsequently analysed a large number of chlorides of different and the constitution of the fluorides and origin, investigated " " double fluorides of niobium. A hyponiobic fluoride was found to contain three atoms of fluorine for one atom of oxygen, and its double fluorides with the fluorides of other metals were shown to be isomorphous with similar double fluorides given by titanium tetrafluoride,

TiF 4 , tin tetrafluoride, SnF 45 and tungsten oxydifluoride, WO 2F 2 . Furthermore, a comparison of the oxyfluorides of tungsten with the fluorides of titanium, tin, silicon and zinc had previously shown that an atom of fluorine in these compounds could be replaced by an atom of oxygen without disturbing the isomorphism. The formula for " " fluoride became and in hyponiobic thereupon NbOF 3, consequence, niobic anhydride became Nb 2O5 . The formula for tantalic anhydride was established as Ta 2O5 at the same time, because tantalic acid occurs in isomorphous mixtures with niobic acid in several minerals, from which isomorphous double fluorides, such as potassium tantalum fluoride, NbF were also obtained. K 2TaF7 , and potassium niobium fluoride, K 2 7 , These conclusions were confirmed by determinations of the vapour tantalum densities of niobium pentachloride, NbCl5 , pentachloride, Deville and Troost in TaCl5 , and niobium oxytrichloride, NbOCl 3, by 1865. 2 3 Marignac also showed that previous methods for the separation of niobium and tantalum were far from perfect, and for the first time he succeeded in preparing pure niobium and tantalum compounds. His methods are still in use to some extent, and his analyses provided the first reliable values for the atomic weights of these elements. It should be stated, too, that Rose's earlier researches, which extended over a period of nearly twenty years, have provided a valuable source 4 of information for the chemistry of niobium and tantalum. His 5 calculations and formulae were revised by Rammelsberg in the light of subsequent discoveries* During the years 1860 to 1871 the presence of various other elements 6 in columbites and tantalites was reported. Hermann defended the original formulae of Rose for niobium and tantalum compounds, and announced the existence of another element, which he styled ilmenium, but in yttro-ilmenite, mmarskite, and other niobium-bearing minerals, ilmenium was shown by Marignac to be a mixture of niobium and titanium. Hermann also claimed 7 the discovery of still another metal, 8 which he called neptunium, but Blomstrand and Larsson independently 9 showed it to be identical with niobium, and Smith was unable to 10 to the conclusion that confirm Hermann's preparation. Kobell came niobites from Bodenmais in Bavaria contained the acid oxide of a metal which he named dianium, and which was not present in niobites of dianium and from North America and Greenland ; but the identity

1 49 249. Marignac, Ann. Chim. Phys., 1866, [iv], 8, 5, ; 1866, 9, 2 Deville and Troost, Compt. rend., 1865, 60, 1221. 3 4 1844-63, Marignac, ibid., 1865, 60, 234. Rose, Pogg. Annalen, 63-118. 6 Bamnielsberg, ibid., 1869, 136, 177, 362. 6 108 373 ; 1871, 4* 178 ; Hermann, /. praM. Chem., 1846, 38, 91 ; 1870, 2, ; 1871, 3, 59. Chem. News, 1871, 24, 73 ; 1872, 26, 194 ; 1873, 27, 7 Chem. 197. Hermann, Zeitsch. anal Chem., 1871, 10, 344 ; News, 1877, 3& 8 Larsson, Zeitsch. anorg. Chem., 1896, 12, 189. 8 209. E. F. Smith, Proc. Amer. Phil 800., 1905, 44 151 ; Chem. News, 1905, 92, 10 449 Chem. News, 1862, 5, 41. Kobell, J. prakt. Ghem., 1860, 79, 291 ; 1861, 83, 193, ; 124 VANADIUM, NIOBIUM, AND TANTALUM.

1 2 niobium was also soon established. Kruss and Nilson, on spectrp- existence of still another element in scopic evidence, assumed the untenable.3 A fergusonite, but the assumption became re-investigation 4 to of American nio bites and tantalites carried out in 1905 failed to bring

and tantalum ; this result light any new elements other than niobium of several of niobium was confirmed by spectroscopic examination samples various minerals. 5 pentoxide which had been prepared from Niobium and tantalum suddenly received considerable attention the 1905 as materials for the filaments of incan- about year possible ^ filament then in use. descent electric lamps in place of the carbon the first time The metals were then prepared in the pure state for 6 were examined. by Dr Werner von Bolton, and their properties Niobium was found to be unsuitable for the purpose in view, but were manu- tantalum proved to be satisfactory. Tantalum lamps 1905 and when the factured in large quantities between the years 1911, metal was displaced by the electrically more efficient tungsten. Extraction. The method of extraction of niobium and tantalum differ from the compounds from their natural ores does not process the and consists, followed in the quantitative examination of ores, an alkali or alkali salt, briefly, in fusing the material with extracting the fused mixture of niobates and tantalates with water, and hydrolysing insoluble mixture of the solution by boiling, whereupon a comparatively niobic and tantalic acids or their anhydrides is obtained, which yields those metals the Nb 2 5 and Ta 2 5 , on being ignited. Only pentoxides, 7 which give rise to acid oxides demand special separation. The more detailed description of the extraction is conveniently divided into three stages : man- I. Preparation of a mixture free from tin, antimony, iron, ganese, etc. II. Removal of titanium from the mixture. III. Separation of niobium from tantalum in the product.

I. Preparation of a Mixture Free from Tin, Antimony, Iron, Manganese, Etc. should contain as The finely powdered niobitc or tantalite, which excess little titanium as possible, is fused for several hours with a large a of potassium hydrogen sulphate or sodium hydrogen sulphate in silica or crucible the cooled mass is extracted platinum ; thoroughly of by boiling with water, and the precipitate, which consists mainly 8 niobic and tantalic acids together with some of the sulphates, is digested with ammonium sulphide to remove tin, antimony and tungsten, and to convert any iron or manganese into sulphide, which 1 Devflle and Troost, Compt. rend., 1861, 53, 1044. 2 86. Kruss and Nilson, Ber. y 1887, 20, 2134 ; Ghem. News, 1887, 56, 3 Larsson, loc. cit. 4 CJiem. E. F. Smith, loc. cit. ; Hall and Smith, Proc. Amer. Phil 8oc., 1905, 44, 177 ; J. CJietn. 1235. News, 1905, 92, 220, 232, 242, 252, 262, 276 ; Hall, Amer. 8oc., 1904, 26, 1672. Barr, ibid., 1908, 30, 1668 ; Hildebrand, ibid., 1908, 30, 8 see also von Bolton, Zeitsch. Elektrochem., 1905, II, 45, 722 ; 1907, 13, 145 ; Siemens, Chem. News, 1909, 100, 223. 7 For a scheme for the complete analysis of an ore, see Schoeller and Powell, The Analysts of Minerals and Oreft of the Rarer Elements (Griffin, London), 1919, 141. 8 J. Chem. Soc. Todd, Univ. Toronto Studies, Geol Ser., 1923, No. 16, pp. 40-45 ; 9 Abs., 1924, ii, 126, 207; Giles, Ohm. News, 1909, 99, 3. NIOBIUM AND TANTALUM. 125 is removed with hydrochloric acid.1 Some of the niobic and tantalic acids may also be dissolved by the hydrochloric acid, however. 2 Separation of silica is effected in the usual way by evaporation of the hydrofluoric acid solution of the residue in a platinum dish with addition of sulphuric acid. In order to remove silica without loss of niobium and tantalum through possible vaporisation of the penta- fluorides, NbF5 and TaF5 , extraction of the niobic and tantalic acids with caustic soda or caustic potash has been recommended.3 Treatment of the fused ore with ammonium sulphide, as described above for the removal of tin, antimony and tungsten, does not proceed better is quantitatively ; separation claimed to result on fusing the mixed niobic and tantalic acids with a large excess of a mixture of sodium carbonate and sulphur and then extracting with water, several 4 refusions being necessary. An alternative method for the removal of tungsten consists in boiling the alkaline aqueous extract of the fused ore with ammonium nitrate, the mixed niobic and tantalic acids being 5 precipitated, while ammonium tungstate is left in solution. In a more recent method the mixed niobic and tantalic acids are fused with potassium carbonate and the aqueous extract treated with sodium chloride ; the mixed acids are thereupon precipitated, the tungsten 6 being left in the filtrate as tungstic acid. Instead of lixiviating with water, the pyrosulphate fusion is followed 7 the in a recent process by extraction with tartaric acid solution ; insoluble residue contains silica, tin, and lead, and the solution, after being saturated with hydrogen sulphide for the precipitation of copper, antimony, etc., contains the hydroxides of niobium and tantalum as well as tungsten, titanium, zirconium, rare earth metals, etc. In addition to potassium hydrogen sulphate and sodium hydrogen 8 9 sulphate for opening up the ore, potassium carbonate, sodium 10 peroxide, and alkali hydroxides have been employed. The use of potassium hydroxide is preferred in the case of a high-grade ore of low n it over sodium that titanium content ; has the advantage hydroxide potassium tantalates are soluble in solutions which contain excess of the alkali, whereas sodium tantalates are insoluble. For minerals in which the titanium content is high it has been found preferable to attack the ore with potassium hydrogen fluoride, KHF 2 , 12 or concentrated hydrofluoric acid. In one such process the powdered

1 62 J. Chem., Marignac, Ann. Chim. Phys., 1866, [iv], 8, ; Rammelsberg, prakt. 64 Weinland and Zeitsch. 1869, 107, 343 ; Pogg. Annalen, 1871, 144, ; Slorz, anorg. 1132. Chem., 1907, 54, 230 ; Chesneau, Compt. rend., 1909, 149, 2 Chem. 3. Weiss and Landecker, Zeitsch. anorg. Chem., 1909, 64, 65 ; News, 1910, 101, 3 and von John, Chem, News, 1909, 100, 154 ; compare SchoeUer Powell, Analyst, 1928, 53, 258. 4 504 loc. cit, Compare, however, Schoeller and Jahn, ibid,, 1927, 52, ; Giles, J, Amer. Chem, 26, 1235 ; E. F. Smith, Chem, News, 1905, 92, 209 ; Hall, Soc., 1904, Bedford, ibid,, 1905, 27, 1216. 6 Chem. 184 ; Bullnheimer, Chem. Zeit,, 1900, 24, 870 ; News, 1902, 85, compare Schoeller and Jahn, loc. cit. 6 Schoeller and Jahn, loc. cit. 7 Powell and Schoeller, J, Chem, Soc., 1921, 119, 1927. 8 122. Sears, J. Atner. Chem. Soc., 1926, 48, 343 ; 1929, 51, 9 Chem. 1, 25. Russ, Zeitsch. anorg. Chem., 1902, 31, 50 ; Giles, News, 1909, 99* 10 Chem. 1129 ; Rose, Moir, ibid., 1916, 113, 256 ; Balke, J, Amer. Soc., 1910, 32, Pogg. Annalen, 1861, 113, 301. 11 Industrial and Chemistry, Simpson, Chem. News, 1909, 99, 243 ; Balke, Engineering ia Chem, 13, 29. 1923, 15, 560. Smith, Compt. rend., 1878, 87, 146 ; News, 1883, 48, 126 VANADIUM, NIOBIUM, AND TANTALUM. with a solution of niobite is evaporated almost to dryness potassium and dissolved in hydrogen fluoride, and the residue fused hydrofluoric tantalum fluoride, Iv 2 rai 7 , acid, from which crystals of potassium .H 0, are obtained on and of potassium niobium oxyfluoride, K 2NbOF5 2 freed from iron and manganese evaporating and cooling. These are treatment m solution either by recrystallising, or better, by previous 1 with hydrogen sulphide. . of historical interest, Older methods of opening up the ore, now only a mixture of charcoal consisted in heating it for several hours with sugar and sodium carbonate in a carbon crucible, whereupon the niobium, tantalum and titanium formed their carbides and nitrides. The pro- acid and dilute duct was treated with boiling concentrated hydrochloric the calcium, and some of hydrofluoric acid, which removed tin, iron, heated in a stream of the yttrium. The dried residue was carefully titanium and silicon were chlorine; the more volatile chlorides of tube a mixture of the chlorides thereby removed, and there was left in the of feme of niobium and tantalum, together with small proportions was extracted with chloride and tungsten oxychloride. The product and tantalum chlorides - dilute hydrochloric acid, and the niobium hydro 2 Moissan 3 obtained a mixture lysed by boiling water to the pentoxides. of niobic and tantalic acids very conveniently by heating a powdered Most of the man- niobite with sugar charcoal in the electric oven. and silicon were volatilised ; ganese and the greater portion of the iron and the residue, which consisted of the carbides of niobium tantalum, a small of nitric was dissolved in hydrofluoric acid to which quantity acid had been added. The iron was removed with ammonium sulphide, concentration and and potassium fluoride added, whereupon cooling TaF and gave a mixture of potassium tantalum fluoride, K 2 7 , potassium (X niobium oxyfluoride, K 2NbOF5 .H 2 Zirconium can be removed from the mixed precipitated acids by the melt with fusing them with potassium carbonate and extracting cold water. The niobium and tantalum pass into solution as niobate the zirconium remains and tantalate of potassium respectively, while 4 is more suited for the undissolved as the dioxide, Zr0 2 . The method 5 removal of zirconium from niobates than from tantalates.

II. Removal of Titanium. The removal of titanium from mixed niobic and tantalic acids is a difficult matter. Although titanium and niobium compounds display considerable differences in their general behaviour, when the two elements occur together they appear to undergo a change, in consequence of which they become difficult to separate. Niobic acid, for instance, acid is precipitated from a much more concentrated boiling sulphuric acids are dissolved solution than is titanic acid ; but when the two 1 J. Amer. Ghem. Gibbs, Amer. J. Sci., 1864, [ii], 37, 355; Pennington, Soc., 1896, J. 393. 18, 40; Barr, ibid., 1908, 30, 1669; Fpote and Langley, Amer. Sci., 1910, [iv], 30, 2 138. Joly, AnnaUs Scientifiques de V$coU Normale Superieurc, Paris, 1877, [ii], 6, 3 20. Moissan, Bull Soc. cMin., 1902, [iii], 27, 430 ; Gompt. rend., 1901, 133, * ISToyes and Bray, Qualitative Analysisfor ike Rare Elements (Macmillan, London), 1927, J. 481 Ghem. 79, 103 ; see also Bailey, Ghem. Soc., 1880, 49, 149, ; Morozewitz, Zentr., 2311 Harden and U.S. 1909, [i], 1965; Headden, Amer. Chem., Abs. 9 1917, n, ; Rich, Bureau of Mines, Bulletin 186 (1921). 5 Schoeller Schoeller and Powell, /. Ohem. Soc., 1921, 119, 1927 ; see also and Water- Louse, Analyst, 1928, 53, 467. NIOBIUM AND TANTALUM. 127 together in sulphuric acid the precipitation of the niobic acid does not take place unless the solution has been diluted considerably, i.e. the 1 hydrolysis of the niobium salt is impeded by the presence of titanium. (On the other hand, the hydrolysis of the niobium salt is accelerated of a small of tantalum. when titanic by the presence quantity ) Again, acid is fused with potassium carbonate and the melt is extracted with 1 cent, of the titanic acid is dissolved boiling water, only about per ; in the presence of niobic acid, however, the greater proportion of the titanic acid passes into solution on the same treatment, and some of the niobic acid remains in the residue. These modifications in the properties of niobium in the presence of titanium were in part responsible for the erroneous assumption of the existence of various new elements in niobium and tantalum ores (see p. 123). More recently it has also been shown that the solubilities of niobic acid, tantalic acid and titanic acid in acidified hydrogen peroxide solution, are affected by the presence 2 of each other, according to the conditions. A completely satisfactory process for the quantitative separation 3 of titanium from niobium and tantalum has not yet been evolved ; nearly all the methods that have been suggested from time to time have in this subsequently received adverse criticism. One that suffers least acids for several respect consists in boiling the mixed precipitated hours with excess of a dilute solution of salicylic acid. The titanic the residue is and the several acid is dissolved ; ignited process repeated times, when all the niobium and tantalum are contained in the residue, subse- while the salicylate filtrates .contain all the titanium, which is as titanium quently precipitated with ammonia and estimated dioxide, 4 which is stated to be available Ti0 2 . Another process of separation, in in the presence of appreciable amounts of titanium, consists fusing with sodium the mixed acids (after removal of tin, antimony, tungsten) is claimed to nitrate, or with an alkali and an oxidising agent, which of titanium with niobium prevent the formation of soluble compounds 5 a or tantalum. After extracting the fused product with water only this is removed very little titanium remains in solution, and by 6 until free from hydrogen sulphide. The filtrate is boiled hydrogen and boiled with sulphide, acidified with sulphuric acid, sulphurous tantalic acids. 7 acid, which precipitates only niobic and Alternatively, in acidified the mixed precipitated acids may be dissolved hydrogen per- acid the contains a oxide and boiled with sulphurous ; precipitate only is stated to small amount of titanium, and repetition of the process yield 8 acids with a mixture a titanium-free product. Fusion of the mixed 1 Ann. Chim, Schoeller and Waterhouse, Analyst, 1928, 53, 467 ; Marignac, Phys.,

2 Cheat. News, 1907, H^taand" Gille, Zeitsch. anorg. Chem., 1920, 112, 283; Giles, 95, 37. s and 1927, 625. Schoeller Deering, Analyst, 52, /, ,. * i * and Qualitative Analysis Muller, J. Amer. Chem. Soc., 1911, 33, 1506 ; Noyes Bray, 77 Dittrich and Freund, Zeitsch. for the Hare Elements (Macmillan, London), 1927, ; anorg. Chem., 1907, -56, 344, 346. . 100 Meim- s Compare, however, Hauser and Lewite, Zeitsch. angew. Chem., 1912, 25, ; berg and Winzer, ibid., 1913, 26, 157. * this. Ruff and Schiller (Zeitsch. anorg. Chem., 1911, 72, 329) deny ' Chem. 101, 14 ; compare Weiss and Landecker, ibid., 1909, 64, 65 ; News, 1910, and Maas, Zeitsch. angew. Chem., 1910, Ott, Dissertation (Munich, 1911) ; Wedekind 23 ' obtain 8 cit. and Gille were unable to We"iss and Landecker, he. ; Hahn (loo, cit.) satisfactory separation by this method. 128 VANADIUM, NIOBIUM, AND TANTALUM. of caustic potash and potassium cyanide has also been recommended of titanium the melt is extracted for the quantitative separation ; 1 with hot water, all the titanium remaining in the residue. 2 In 1905 Hall and Smith investigated all the then known methods for the removal of titanium, and tried various other processes ; they were unable, however, to improve on Marignac's method of fractional 3 method has recrystallisation of the double potassium fluorides. This the disadvantage that in the case of the niobium salt protracted and tedious repetition is necessary before it is obtained free from titanium, 4 and the method becomes impossible with small quantities of material.

III. Separation of Niobium and Tantalum. The close similarity in the chemical behaviour of the compounds of these two elements has rendered their separation extremely difficult. most in Although many processes have been investigated, the method or use appears to be that evolved by Marignac as long ago as 1866, 5 in the a modification of it. This depends, firstly, on the difference .H solubilities of potassium niobium oxyfluoride, K 2NbOF5 2 (1 part in 12 to- 13 parts of water between 17 and 21 C.), and potassium in 150 to 160 of water con- tantalum fluoride, K 2TaF7 (1 part parts taining a small quantity of hydrofluoric acid, at the same temperature) ; and, secondly, on the fact that these two compounds are not iso- not rnorphous, and mixed crystals or solid solutions are therefore produced. For the separation, the mixture of niobic acid and tantalic acid is dissolved in concentrated hydrofluoric acid, potassium fluoride 6 is added in correct quantity, and the whole is carefully concentrated. The double potassium tantalum fluoride is first precipitated in acicular, with further rhombic needles ; the filtrate, on being concentrated, addition of hydrofluoric acid and potassium fluoride, yields white, semi-transparent, granular plates of potassium niobium oxyfluoride mixed with the needles of potassium tantalum fluoride, which are separated by recrystallisation. In a modification of this process, potassium chloride is added to the hydrofluoric acid solution of the niobic and tantalic acids instead of potassium fluoride; the double dissolved and the tantalum potassium niobium fluoride, K aNbF7 , remains 7 salt is precipitated. Ruff and Schiller 8 determined the solubilities of the double fluorides in of niobium and tantalum, K 2NbF7 and K 2TaF7 , varying quantities of hydrofluoric acid and potassium fluoride, and based a method of fractional separation on the results, which showed that the solubility of both fluorides diminishes with increasing concentration of potassium fluoride and decreasing concentration of hydrofluoric acid; the solubility increases rapidly with rising temperature, and is always

1 Weiss and Landecker, loc. cit ; Moir, loc. cit. 2 Hall and Smith* Ghem. News, 1905, 92, 232. * Marignac, Ann. Chim. PJiys., 1866, [iv], 8, 5, 49, 68. 4 Kruss and Nilson, Ber. t 1887, 20, 1684 ; Smith, Chem. News, 1905, 92, 209. 5 loc, cit. Marignac, ; see also Ruff and Hchiller, Zeitsch. anorg. Chew,., 1911, 72, 239 ; Meiniberg, Zeitech. angew. Cfiem., 1913, 26, 83 ; Levy, Analyst, 1915, 40, 204. 6 For practical details see Mellnr, A Trcatue, on Quantitative Inorganic. Analysis (Griffin, London), 1913, 421 ; also Schoeller and Powell, The Analysis of Minerals and Ores of the Rarer Ekments (Griffin, London), 1919, 133. 7 Meiniberg and Winzer, Zeitsch. angew. Chem., 1913, 26, 157. 8 Ruff and Schiller, loc. cit. NIOBIUM AND TANTALUM. 129 greater for the niobium than for the tantalum salt. The tantalum l 2 may also be precipitated and removed partly or completely as potassium tantalum oxyfluoride, while the niobium remains in solution.* Conversion of the separated fluorides into the corresponding oxides is effected by boiling with concentrated sulphuric acid until free from fluorine, and then hydrolysing the product by boiling -with water. Alternatively, the hydrated acids are precipitated by the addition of 4 ammonia to the solutions of the double fluorides. Niobium pentoxide, O is obtained on of the Nb 2O59 or tantalum pentoxide., Ta 2 5 , ignition precipitated hydrate. A method of separation which avoids the preparation of the double fluorides consists in fusing the mixed niobic and tantalic acids with is sodium carbonate and nitrate ; the product digested with warm water and a current of carbon dioxide is passed through the solution* 5 It is claimed that only tantalic acid is precipitated. This process has, 6 however, been the subject of adverse criticism. Partial separation of niobium from tantalum can be effected by warming the mixed, with a mixture of freshly precipitated, hydrated oxides hydrogen while peroxide and hydrochloric acid; the niobium dissolves readily, 7 the tantalum dissolves only sparingly. A more recent process, which avoids the difficulties associated with and Marignac's method, is based on the solubility of niobium pentoxide in mixture of the comparative insolubility of tantalum pentoxide a SeOCI and concentrated equal volumes of selenium oxychloride, a, 8 is left in the and sulphuric acid. The tantalum pentoxide residue, niobic acid. hydrolysis of the extract after dilution yields

Estimation of Niobium and Tantalum.

The various methods in use for opening up the natural ores, and the from niobium and tantalum, have separation of other metals already elements been described in dealing with the extraction of these (see p methods to The quantitative 124) ; similar apply quantitative processes. determination of either niobium or tantalum is best effected by con- into the and verting the niobium or tantalum compound pentoxide or double weighing as such. In the case of the fluorides, oxyfluorides, the fluorides with the alkali metals, and the niobates and tantalates, conversion is effected either by digesting with concentrated sulphuric the after acid or by fusing with potassium hydrogen sulphate ; residue, of ammonium carbonate- extraction with water, is ignited in the presence , contain both niobium In technical practice the product will usually pentoxide and tantalum pentoxide. * 100, 154. * Hall and Smith, Chem. News, 1905, 92, 221. von John, iM., 1909, 3 * Chem. Met. 1922, 26 12,2. and Bray, fax oft. Bafce, Eng Noyes Chem. 1910, 101, * Weiss and Landecker, Zeitsch. anorf. Chem., 1909, 64, 65; News, 2 1^1 26 ' Hauser and Uwite * Vedefcind and Maas, Zeitsch. anyew. Chem., 1910, 23, 2314 ; Amer. J.8a. f 1910, [iv], 3<>, 401 ? Meunberg ibid., 1912, 25, 100 ; Foote and LangLey, also loc. cit. and Winzer, foe. cit. ; compare Headden, 7 Weiss and Landecker, fee, cit 21 Merrill, Art., 1921, 43, 2378, Lenher, /. Amer. Chem. oc. 9 1921, 43, ; ' Aw,. Gfam. JPhy*., 1866, [iv], Wo'hler, Pogg. Annaten, 1839, 48, 92 ; Marignac, Weiss and Landecker, Zeitscb. anorg, Chem-, 1909, 64, 66, 8y 63 ; " TOL.VI.: in. 130 VANADIUM, NIOBIUM, AND TANTALUM.

Estimation of Niobium and Tantalum when Present Together.

(a) Gravimetric Methods. The mixture of pentoxides of niobium and tantalum is redissolved in concentrated hydrofluoric acid the and separated by Marignac's process (see p. 128) ; potassium tantalum fluoride and potassium niobium oxyfluoride are then separately 1 converted into the pentoxides as described above, and weighed. This

: ratio of the solubilities of method has several disadvantages (1 ) The the two compounds on which the separation is based is only approxi- inaccurate mately 10 : 1, and the process is, therefore, necessarily ; even when the recrystallisation is repeated to a tedious extent the error approaches 1 per cent. (2) The concentration of the hydrofluoric fluoride be controlled acid and of the potassium must carefully ; if is is if the acidity too low, an oxyfluoride of tantalum precipitated ; the acidity is too great, a normal fluoride of niobium is obtained. (3) Several platinum dishes are necessary. An entirely different method of separation, which avoids the disadvantages of Marignac's process, is based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly acid solution. The colour of the tantalum precipitate (sulphur yellow) is much paler than that of the niobium 2 precipitate (vermilion). The presence of titanium interferes with the 3 precipitation. Extraction of niobium pentoxide in a mixture of equal volumes of selenium oxychloride and concentrated sulphuric acid, in which tantalum pentoxide is insoluble, also provides a convenient 4 quantitative separation. In another recent process the niobium pentoxide is determined in the presence of tantalum by reducing it to the NbO in a stream of in dioxide, 2, hydrogen, and noting the gain 5 weight on reoxidising it in air at a^red heat. A rough method for the estimation of niobium pentoxide and tantalum pentoxide depends on the considerable difference in their densities the ; ordinary specific gravity bottle is used, and the composition of the mixture ascertained by reference to a table.6 The analysis of ferrotantalum alloys and of tantalum steels also involves the conversion of the tantalum present into the pentoxide. *The material is dissolved in hydrofluoric acid and nitric acid, evaporated to and the residue fused with dryness, potassium hydrogen sulphate ; extraction with dilute hydrochloric acid and hydrolysis yield a precipitate of hydrated tantalic pentoxide, the iron remaining in solution. 7 Cupferron can be employed for the estimation of niobium and tantalum together, but does not differentiate between them; any titanium is 8 present also simultaneously precipitated. Volumetric (fc) Methods. Pentavalent niobium compounds differ 1 Compare Tighe, J. Soc. Ghem. Ind., 1906, 25, 681. Powell and Schoeller, Analyst, 1925, 50, 485. Schoeller and Powell, ibid., 1928, 53, 258. Merrill, J. Amer. Chem. Soc., 1921, 43, 2378. Ruff and Thomas, Zeitsch. anorg. Chem., 1926, 156, 213. Foote and Langley, Chem. News, 1911, 53 103, ; Levy, Analyst, 1915, 40, 216 ; remarks on compare, however, the variation of the specific gravities of these oxides with heat treatment, pp. 155 and 197. 7 Arnold and Ibbotson, Steel Works Analysis (Pitman, London), 1919, 212, 265 ; Travers, rend., 494 Oompt. 1918, 166, ; Kelly, Myers, and Illingworth, J. Ind. Eng. Ghem., 1917, 9> 852. 8 Pied, Compt. rend., 1924, 179, 897. NIOBIUM AND TANTALUM. 131 from pentavalent tantalum compounds in that they can be reduced by nascent hydrogen in hot acid solution approximately to the trivalent state, and can then be titrated back with potassium permanganate solution, the tantalum compound remaining unaltered. The final stage of the reduction has been variously reported as being equivalent to 1 2 3 Nb O Nb . and to NbiAa, 10 17 , 2 3 107 , appears depend on the quality of the reagents, the degree of dispersion of the niobic acid, and on other conditions concerning which insufficient is known at present to render the method accurate.4 Treadwell 5 recommends reduction with cadmium amalgam in the presence of ammonium vanadate, ammonium molybdate, or titanium sulphate, and electrometric titration of the trivalent solution so obtained with potassium permanganate. (c) Colorimetric Method. A colorimetric method for the estimation of small quantities of niobium in tantalum compounds has been 6 worked out by Meimberg. This takes advantage of the colour change that is produced when a pentavalent niobium salt in acid solution is

reduced with tin ; a tantalum salt remains unaffected under the same conditions. The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K4Ta4 5F14 (see p. 132), is thrown solution of 7 down when a potassium tantalum fluoride, K 2TaF7 , is boiled. 8 Powell and Schoeller find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo- niobic acid and oxalo-tantalic acid in the presence of tannin in slightly acid solution) for the detection and estimation of traces of tantalum in niobium compounds. The determination of tantalum by ordinary methods of spectro- 9 and means of be scopy by X-ray sp^^-afep ^jpearg^foo possible.

of 1 almost on The detection niobium and tantalurlHfte^ entirely 4 ^ the reactions given by niobic acid and tantaffc axM All the common niobium and tantalum compounds are hydrolysed on being boiled in acid solutions, and yield precipitates of the respective acids. Natural minerals are previously fused with potassium hydrogen sulphate, and the aqueous extract of the melt usually precipitates the mixed acids 1 298. Osborne, Chem. News, 1886, 53, 43 ; compare Warren, ibid., 1906, 94, 2 Levy, toe. cit, 3 1 Metzger and Taylor, Chem. News, 1909, 100, 257, 270 ; Giles, ibid,, 1909, 99, ; J. Todd, Univ. Toronto Studies, Geol. Ser., 1923, No. 16, pp. 40-45 ; Chem. $oc., Ato., 1924,

126, [ii], 207. 4 Schoeller and Waterhouse, Analyst, 1924, 49, 215; compare Kiehl aud Hart, J. Amer. Chem. Soc., 1928, 50, 1608. Treadwell, Helv. Chim. Acta, 1922, 5, 732, 806. Meimberg, Zeitech. angew. Chem., 1913, 26, 83. Hall, J. Amer. Ohem. Soc., 1904, 26, 1239. Powell and Schoeller, Analyst, 1925, 50, 485. Powell and Schoeller, toe. cit. * 10 Hevesy and B6hm, Zeitsch. anorg. Chem., 1927, 164, 69. 11 Weiss and Landeeker, ibid., 1909, 64, 100 ; Chem. News, 1910, 101, 28 ; Moir, #& 7. .i8 52 Chem. 1916, 113, 256 ; Pennington, Amer. Gkem. Soc., 1896, ; News, 1897, 75, 31. 132 YANADIUM, NIOBIUM, AND TAOTALIJM.

removal of other metals is effected by the methods spontaneously ; described on p. 124 et seq. tantahc acid dissolve A. Wet Reactions. (1) Both niobie aeid and in concentrated readily in hydrofluoric acid, but only very slightly acid. The residue hydrochloric acid and in hot concentrated sulphuric from the hydrochloric acid solution of niobie acid, however, readily The acid forms a hydrosol on being triturated with water. sulphuric solution of niobie acid remains clear on being diluted with water, of tantalic acid becomes turbid on whereas the sulphuric acid solution being diluted, and reprecipitates the acid. acid does not a The hydrofluoric acid solution of niobie yield (2) niobium precipitate on the addition of potassium fluoride (potassium is soluble in about 12-5 of fluoride, K 2NbF 7 , which formed, being parts solution of tantalic water), whereas the hydrofluoric acid acid^ yields tantalum K TaF colourless, rhombic needles of potassium fluoride, ? 7 under the same (which is soluble in about 150 parts of water conditions), when treated with a saturated solution of potassium fluoride, carefully removal of the and evaporated and cooled slowly. After tantalum, of with further concentration, any niobium present separates in plates

.H O 3 if the acid potassium niobium oxyfluoride, K 3NbOF 5 2 hydrofluoric is not in excess, and in needles of potassium niobium fluoride, K 2NbF7 , if the hydrofluoric acid is in excess. is a fusible The potassium tantalum fluoride first precipitated a substance. Its aqueous solutions on being boiled precipitate very O .2TaF or insoluble potassium tantalum oxyfluoride, 4KF.Ta 2 5 5 This reaction is stated to constitute K4Ta4 5F145 as a white powder. a sensitive test for tantalum. 1 niobium (3) Hydrochloric acid solutions of pentavalent compounds which are free from hydrofluoric acid, on being reduced with zinc, first become blue, and with further action of the reducing agent, olive-green or dark brown, according to the concentration of the acid and other difficult to attain the brown conditions. The blue stage is not ; stage reductor several is best attained by passing the solution through a zinc times. The reduced solution precipitates white mercurous chloride from solutions of mercuric chloride. This reaction is given by 1 mgm. of niobium. Tantalum compounds in solution do not give a colour change on being reduced with zinc, and this test also serves to establish niobium 2 in the presence of titanium, which produces a violet coloration. Vanadium, molybdenum, and tungsten solutions, however, behave similarly to niobium, and these metals must, therefore, be previously removed. (4) When a solution of niobie acid in concentrated hydrochloric a colora- acid (2 : 1) is boiled with tin for some time, deep sapphire-blue tion is obtained, which fades on standing and is regenerated by boiling, to an alkaline solu- (5) Addition of excess of potassium thiocyanate tion of a niobate, followed by zinc and concentrated hydrochloric acid, produces a golden-brown colour which may be almost red in the presence of larger quantities of niobium. It is stated that neither tantalum nor 3 titanium gives any coloration under the same conditions. Addition of

1 Compare Powell and SchoeUer, Analyst, 1925, 50, 495. 3 Compare Moir, foe. cit. 3 Pennington, loc. cit. NIOBIUM AND TANTALUM. 133 potassium thiocyanate to a hydrochloric acid solution of tantalic acid or niobic acid gives a colourless solution. (6) Potassium ferrocyanide yields a yellow or reddish-brown precipitate with a hot solution of tantalic acid in hydrochloric acid ; niobic l 2 acid gives a reddish-brown or greyish-green precipitate. (7) Tannin produces an orange-red or chocolate-red precipitate with an acid solution of niobic acid, and a yellow or light brown precipitate with acid solutions of tantalic acid. Pyrogallol and other polyhydroxy derivatives of benzene behave similarly. (8) Addition of ammonium hydroxide or ammonium sulphide to solutions of niobic acid and tantalic acid in mineral acids reprecipitates the niobic and tantalic acids, which may, however, retain some of the ammonia. This test does not distinguish between niobium and tantalum, and it does not proceed in the presence of tartaric acid. B. Dry Reactions. When heated in the reducing flame a bead of microcosmic salt assumes a blue, violet, or brown colour with increasing quantities of niobic acid ; the heated bead becomes red on the addition of ferrous sulphate. With tantalic acid under these conditions the bead remains colourless. Borax beads do not produce colorations either with niobium or tantalum.

1 Moir, loc. cit. 2 loc. cit. on effect of Weiss and Landecker, ; compare also Pennington (loc. cit.) presence of fluorides. CHAPTER VI. NIOBIUM AND ITS ALLOYS.

Symbol, Nb. Atomic Weight, 93-3 (O=16). Preparation of Metallic Niobium. The preparation of metallic niobium is not carried out industrially, as there is no demand for the metal. Its laboratory preparation depends on the reduction of the pentoxide. This is effected with difficulty, firstly, because of the 1 tendency for partially reduced products to be formed, and secondly, 2 because of the tendency of the reducing agent, for example carbon 3 or aluminium, to combine* or alloy with any niobium formed. One method of preparation consists in a modification of the Gold- schmidt process. Niobium pentoxide is mixed with an alloy of the rare earths, called mixed metal, obtained in the manufacture of thorium and of 45 cent, of cerium, 20 nitrate, consisting roughly " per per cent, of lanthanum, 15 per cent, of didymium," and about 20 per cent, of other rare-earth metals. The reaction is carried out in a magnesia-lined crucible, and is started with a firing mixture of barium peroxide, potassium chlorate, and aluminium powder. Considerable is evolution of heat takes place and the reduction extremely rapid ; 4 a button of niobium is obtained which, however, is not pure. Carbon and aluminium were successfully employed as- the reducing 5 agents in the following special manner. Pure niobium pentoxide was moulded into filaments about half a millimetre in diameter with the aid of a little paraffin, and these were reduced to the tetroxide by heating to whiteness for four or five hours in carbon powder. The filaments so obtained were then heated to whiteness in a vacuum by means of an alternating current, whereupon rapid reduction to the pure metal took place. In order to prepare larger quantities of niobium the pentoxide is first reduced with aluminium powder, and the product, which contains about 3 per cent, of aluminium and some unchanged oxide, is heated in the electric arc in a vacuum until all the impurities are vaporised (a current of 185 amperes at 40 volts for fifteen hours is required for 20 grams of metal). Perfectly pure niobium obtained in this manner has been used for the investigation of the properties of the metal. 1 Rose, Fogg. Anncden, 1858, 104, 310. 2 183 Deville, Compt. rend., 1868, 66, ; Moissan, ibid., 1901, 133, 20 ; Larsson, Zeitsch. anorg. Ghem., 1896, 12, 189. 8 180 loe, cit. Marignac, Com&t. rend., 1868, 66, ; Moissan, ; compare Goldschmidt and Vautin, J. Soc. Chem. Ind., 1898, 17, 543. 4 Weiss and Aichel, Annalen, 1904, 337, 385 ; Muthmann, Weiss, and Riedelbauch, ibid., 1907, 355, 64. 5 von Bolton, Zeitech. Elektrocfam., 1905, u, 45; 1907, 13, 145. 134 * NIOBIUM AND ITS ALLOYS. 135 Niobium pentoxide has been reduced to the metal by means of at 7 and at 1910 1 hydrogen atmospheres pressure C., and a fairly pure sample has been obtained by the action of hydrogen on niobium pentachloride at a red heat. 2 Colloidal Niobium. Sols of niobium have been prepared by sparking electrodes of the metal immersed in isobutyl alcohol by means of an induction 3 coil, or by reducing solutions of niobium salts with hydra- 4 zine, formic acid or formaldehyde in the presence of gelatose. Physical Properties of Niobium. Niobium is variously described as a dull 5 steel 6 7 grey, grey, or white metal with a yellowish tinge, giving 8 a silver-white fracture. The metallic lustre is remarkably permanent, and is not removed even by prolonged boiling with aqua-regia. After being fused in vacuo, rhombic crystals several millimetres long are 9 formed. The crystal structure of the metal has been studied. 10 The density of the fused metal is 12-7 or 12-75 after being rolled into thin 11 foil. Other figures which have been obtained for less pure samples ' are much 12 13 7-06 14 7-37. 15 lower, e.g. 8-4, 7-8, s The hardness of the pure metal is about the same as that of wrought iron, and it will not scratch 16 glass or quartz, but the presence of small quantities of carbon, alu- or increases minium, oxygen the hardness considerably ; a specimen 17 containing about 3 per cent, of carbon scratched quartz easily, It is not very brittle. It can be hammered into foil 0-05 mm. thick, and it is possible, although difficult, to draw it into wire. It can be welded by hammering at a red heat. 18 The specific heat between 21 and 100 C. is 0*071, giving 6-61 for the atomic heat, which figure is in conformity with the law of Dulong 19 and Petit. von Bolton's sample melted at 1950 C. in vacua, but a 20 more recent determination gave the melting-point as 1700 C. 21 Niobium displays weak paramagnetism. The electrical resistance of pure niobium wire, 1 metre long and 1 sq, mm. cross-section, is 0*187 metal ohm ; this figure increases with rising temperature. The volatilises and scatters comparatively easily when made to glow in a vacuum, and is therefore unsuitable for use as the filament in electric lamps.22 Optical Properties. The refractive index of niobium is 1*80, the

1 Wartenburg, Broy, and Reinicke, Zeitsch. EUktrochem., 1923, 29, 214; compare Newbery and Pring, Proc. Roy. Soc., 1916, A, 92, 276. 2 Roscoe, Chem. News, 1878, 37, 25. * Svedberg, Ber., 1906, 39, 1705. 4 Pat. French Pat. 371799 German Pat. 281305 (1913) ; see also Eng. 25864 (1906) ; (1906). 5 c von Bolton, fee. cit. Roscoe, fee. ciL 7 Muthmann, Weiss, and Rdedelbauch, fee. cit. 8 9 Weiss and Aichel, fee. cit. von Bolton, fee. cit 10 617. von Olshausen, Zeitsch. Kryst. Min., 1925, 61, 463 ; Sonder, ibid., 1922-23, 57, 11 von Bolton, fee. cit. 12 Muthmann, Weiss, and Riedelbauch, fee. cit. 1S Baur, ZeitscL physical. Okem., 1911, 76, 569. 14 ** ie Eoscoe, fee. cit. Marignae, fee. cit von Bolton, fee. cit. 17 Moissan, fee. cit. 18 von Bolton, loo. cit. 19 0-0617 von Bolton, fee. cit. An earlier determination with an impure specimen gave (Muthmann, Weiss, and Riedelbauch, fee. cit.}. ao 67. Guertler and Pirani, Zeitsch. fur Metallkunde, 1919, II, 1 ; 1920 9 12, 21 664 ; Loring, Ohem. News, 1914, 109, 122 ; Owen, Ann. Phytik, 1912, [iv], 37, 6 324. Honda, ibid., 1910, [iv], 32, 1027 ; Meyer, Wied. Annakn, 1899, 22 von Bolton, fee. tiL 136 VANADIUM, NIOBIUM, AND TANTALUM. 41-3 cent, coefficient of absorption 2*11, and the reflexion capacity per 1 when measured with yellow light of wave-length A5790. The arc spectrum of niobium has been measured in the region from A=2600 to A6000, using specimens of the pentoxide obtained 2 several niobium ores and a concave grating of 6 feet focus. The more intense lines are tabulated in the following table : ARC SPECTRUM OF NIOBIUM.

The arc and spark spectra in the ultra-violet region have also been 3 photographed and measured. The spectral structure of niobium resembles that of vanadium, and various regularities have been dis- 4 covered in it. In order to be able to establish spectrographically the 1 Wartenberg, Verb Deut. phy&ikal Gee., 1910, 12, 105. 2 HiJdebrand, J. Amer. Ohem. 8oc., 1908, 30, 1672. 3 McLennan and Liggett, Trans. Roy. Soc. Canada, 1926, 20, 377. 4 Jack, Proc. Irish 4=2 Eoy. Acad., 1912, A, 30, ; Paulson, Physical Zeitsch., 1915,

16, 352 ; J. Acad. Meggers, Washington 8ci> 1924, 14, 442 ; Meggers and Kiess, J. Optical Soc. Amer., 1926, 12, 417 ; Laporte, ibid., 1926. 13, 1. NIOBIUM AND ITS ALLOYS. 137 presence of traces of a foreign element in a substance, de Gramont determined which of the lines in the spectrum produced by a condensed spark discharge are the last to appear as the quantity of foreign element is gradually reduced. These are not necessarily the most intense lines in the arc or spark spectrum. The wave-lengths of the ultimate lines given by niobium in this manner with a crown Uviol l glass spectrograph are : 4101-0, 4079*7, 4059*0, 3358*4, in Angstrom units. 2 Eder and Valenta have measured a large number of lines in the flame spectrum given by niobium pentoxide between carbon electrodes. The flame spectrum of niobium between carbon electrodes consists of 3 a blue cone with a yellowish-green shell. The band spectrum of niobium is composed of a very large number of lines which are comparatively sharp but not strong. 4 The X-ray spectra of niobium have been investigated. Aston was 5 unable to obtain a definite mass spectrum of the metal. The arrangement of the electron groups in the atom has been considered by Lessheim and Samuel. 6 Niobium is not radioactive. 7 8 Chemical Properties. When heated in hydrogen, pure, finely divided niobium is converted into a dark grey powder containing a maximum of 1-12 per cent, of hydrogen, which corresponds to a hydride NbH. An impure sample of niobium absorbed 7-5 per cent, of its weight of hydrogen at a red heat. Niobium does not tarnish in the air at ordinary temperatures. When a compact piece of the metal is gradually heated in air it first becomes yellow, then blue, and finally becomes coated with a brownish-blue film of oxide which hinders further oxidation ; the finely divided metal yields the pentoxide only slowly when strongly heated in air or oxygen. The glowing, finely divided metal decomposes water vigorously, with evolution of hydrogen. Heated in nitrogen at 1000 C. it becomes coated with a nitride. Filings of the metal decompose ammonia at a red heat with formation of a nitride which yields ammonia and niobic acid with caustic potash. The metal is attacked by chlorine at about 200 C. with formation of the penta- bromine the at ; chloride ; gives pentabromide higher temperatures 9 iodine is without action. Sulphur and selenium are absorbed with considerable evolution of heat and the formation of a black sulphide or selenide. The pure metal is insoluble in sulphuric acid, hydrochloric 1 de Gramont, Compt. rend. 1920, 171, 1106; Revue de Metallurgie, 1922, 19, 20; Twyxnan, Wave-kngfo Tables for Spectrum Analysis (Adam Hilger, Ltd., London), 1923, 81. 2 Eder and Valenta, Sitzungsber. K. Alcoa. Wiss. Wien, 1910, [iia], 119, 559. 8 Mott, Trans. Amer. Electrochem. Soc.> 1917, 31, 272, 4 The examination of the X-ray spectra and the bearing of the results on the structure of the niobium atom and atoms of many other metals has been reviewed by Coster (Phil. measurements of the and M series of the Mag. 9 1922, 43, 1070). More recent K, L, X-ray spectra are given in the papers cited. 1C series : Leide, Compt. rend., 1925, 180, Ann. 297 ; see also 1203 ; Zeitsch. Physik, 1926, 39, 686 , Schror, Physik, 1926, 80, Moseiey, Zeitsch. Phil. Mag., 1914, [vi], 27, 708. L series : Coster and Mulder, Physik, 1926, also Phil 498. M series : 38, 264 ; see Friman, Mag., 1916, [vi], 32, Dauvillier, Compt. rend., 1926, 183, 193. 5 Aston, PUl Mag., 1924, [vi], 47, 385. Lessheim and Samuel, Zeitsch. Physik, 1926, 40, 220. 7 147 Levin and Mtech., Strong, Amer. Chem. J., 1909, 42, ; compare Buer, Physical. 1909, 10, 576. 8 loc, cii. von Weiss and Aichel, loc. cit. ; Muthmann, Weiss, and Riedelbauoh, ; 20. Bolton, loc. tit. ; Moissan, Compt. rend., 1901, 133, Barr, /. Amer. Chem. Sac., 1908, 30, 1671. 138 VANADIUM, NIOBIUM, AND TANTALUM. acid, nitric acid, and aqua-regia, but it is attacked by these acids when it is alloyed with other metals. It is slowly dissolved by hot hydrofluoric acid, more rapidly in the presence of platinum. Alkali solutions are without action on niobium, but the metal is converted into a niobate when fused -with solid caustic potash, potassium carbonate, etc. Carbon dioxide, sulphur dioxide, phosphorus pentoxide, arsenic pentoxide, chromium sesquioxide, iodic acid, lead oxide, and mercuric chloride are all reduced by niobium when heated with it at high temperatures. Electromotive Behaviour. 1 The electromotive behaviour of niobium is of interest in that it is considerably influenced by cathodic and anodic polarisation. On being immersed in concentrated caustic potash solution or in 10 per cent, ammonium hydroxide, or on being made the cathode in the electrolysis of water, caustic potash, or ammonium chloride for a few minutes, the metal is activated and its potential towards normal potassium chloride increases. On the other hand, on being immersed in concentrated nitric acid, chromic acid, perchloric acid, potassium permanganate, or thiocyanic acid, or on being made the anode in the electrolysis of water, potassium cyanide, chromic acid, or hydrochloric acid, the metal becomes passive and the potential drops to a remarkable extent. Anodically polarised niobium displays valve action (see p. 178) to a pronounced degree. A specimen of niobium which was not very compact (the valve action varies with the degree of compactness), when used as the anode in from 1 to 5 per cent, sulphuric acid, gave only a momentary current with an applied E.M.F. of 112 volts. There was a slight evolution of gas, and the electrode became covered with a greenish-yellow or iridescent blue film which was insoluble in the common acids, but was dissolved by hydrofluoric acid or on making the metal the anode in nitric acid. In an electrolyte consisting of a 0-1 per cent, solution of ammonium phosphate, the E.M.F. necessary to overcome the insulating effect of the oxide-gas layer on the metal is no less than 530 volts. Valve action has also been observed with a large number of other electrolytes. In the cases of hydrochloric acid, sodium chloride, nitric acid, sodium nitrate, acetic acid, potassium bromide and potassium iodide, the anode is dis- integrated and dissolves with formation of the pentavalent niobium followed of niobic compound, by precipitation acid ; hydrofluoric acid acts similarly, but niobic acid is not precipitated. The valve action niobium renders it useful in the displayed by" construction of electrolytic cell rectifiers." An alternating current does not pass through 10 per cent, sulphuric acid when both electrodes are made of niobium even at a pressure of 120 volts; if one of the electrodes is substituted for platinum a unidirectional current is produced.2 Atomic Weight of Niobium. The first determinations were carried 3 4 out by Hermann and Rose, but these are now only of historical interest. 5 In 1864 Blomstrand analysed niobium pentachloride but obtained

1 Muthmann and Frauenberger, jSitzungsber. K. Bayr. Akad. Wiss. Munich, 1904, Ann. pi], 221; Schulze, Phys., 1908, 25, 775; Sborgi, Gazzetta, 1912, [ii], 42, 331 ; Kiehl and Hart, J. Amer. Chem. Soc. 9 1928, 50, 2337. 2 von Bolton, Zeitsch. Ekktrochem., 1907, 13, 145. 3 Hermann, J. prakt. Chem., 1856, 68, 73. * Rose, Pogg, Annakn, 1858, 104, 439. 6 Acta Univ. Blomstrand, Lund, 1864 ; see A Recalculation of the Atomic Weights, Clarke, Smithsonian Institution, 3rd ed., 1910, p. 335. NIOBIUM Am> ITS ALLOYS. 139 discordant figures. They were all higher than the atomic weight of molybdenum, the next higher element in the periodic system, and it seems certain that Blomstrand's material was not free from tantalum. 1 In the following year Marignac published the results of about twenty very carefully conducted analyses of potassium niobium oxy- .H from his data the fluoride, 2KF.NbOF 3 2O ; following figures for the 2 atomic weight of niobium can be calculated : .H 100 , N 2[2KF.NbOFJ 3 20] no ow W - ; hcoceNb.08-87. Nb 2 5 "^36 2KF.NbOF3.Hp -- ^^ ' " -*- K 2S0 4 "57-82 Nb 2 5 _44.-86, ^*>-9370._ (C) K^O, -57-82' " Marignac's material contained traces of titanium, and he therefore assumed that the higher limit would most probably be the more correct, and accordingly suggested an atomic weight of 94. This figure was confirmed by Marignac's analyses of niobium pentachloride and was 3 accepted until 1908, when Balke and Smith redetermined the ratio

2NbCI5 ' Nb 2 5 using material of more reliable purity. The pentachloride was decomposed by water with the aid of a small quantity of nitric acid, and the oxide so produced was ignited and weighed. The mean of nine experiments gave the ratio ~ Nb 2O5 49-805"

This gives an atomic weight of 93-52, which is in harmony with the vapour-density determinations of niobium pentachloride and niobium oxychloride carried out previously by Deville and Troost.* In 1915 5 Smith and Van Haagen criticised the foregoing method on the following grounds : (a) The pentachloride used may have contained traces of the residual oxide have retained traces of chlorine oxychloride ; (&) may ; (c) slight loss of niobium may have occurred, because niobium pentoxide is volatile in hydrogen chloride. These investigators obtained an appreciably lower figure, using a totally different method. Pure sodium metaniobate was decomposed by sulphur monochloride vapour and the residue of sodium chloride was weighed, the niobium being expelled either as chloride or oxychloride. Seven experiments gave the mean ratio

NaNb0 2-80759 , -_ = -- whence Nb= 93*12.ft ;

1 nat. Ann. OUm. 2a Marignac, Arch. Sti. phys. 9 1866, [ii], 23 ; PKys., 1866, [Iv], 8, 16, 2 The fundamental values set out on p. viii of the General Introduction haTO been used. The same fundamental values have been employed in the recalculation of the subsequent values for the atomic weight of niobium mentioned in this section. 3 Balke and Smith, J. Amer. Chem. Soc., 1908, 30, 1645. * Deville and Troost, Compt. rend., 1865, 60, 1221 ; 1863, 5$ 891* 6 Smith and Van Haagen, J. Amer. OJt&m. So&> 1915, 37, 1783. 140 VANADIUM, NIOBIUM, AND TANTALUM. determined the The values for the atomic weight of niobium as by the table : various investigators since 1864 are summarised in following ATOMIC WEIGHT OF NIOBIUM.

The value 93-1 was adopted in 1916 by the International Committee on Atomic Weights; this was altered to 93'3 in 1929.

Alloys. Investigation into the formation of alloys* of niobium with other elements has hitherto been scanty, and even where alloying is known to take place the conditions for the alloy formation and the properties of the products have received little attention. Niobium to with iron in all the 90 appears alloy proportions ; alloy containing 1 per cent, of iron and 10 per cent, of niobium is extremely hard. Nio- bium, usually in conjunction with tantalum because of the difficulty of 2 their separation, can be used for incorporation into special steels. Aluminium-niobium alloys are best produced by the Goldschmidt process. A product which contains about 3 per cent, of aluminium is 3 harder or its is 7*5. brittle of than glass quartz ; density A alloy chromium and niobium is obtained by fusing green chromium oxide 4 and niobium together in the electric furnace. Alloys of niobium and tantalum with nickel and zirconium have 5 also been prepared. It is claimed that the latter can be heated to whiteness in air without oxidation or vaporisation. Potassium, sodium, magnesium, and mercury can be distilled over niobium without of tellurium formation alloys ; arsenic, antimony, and do not form alloys below 500 to 600 C.

1 von Bolton, loc. cit. 2 British Pat. 152371 (1918). * Goldsehmidt and Vautin, J. Soc. Chem. Ind., 1898, 17, 543; von Bolton, kc. tit. Soc. *,Moissan, Compt. rend., 1901, 133, 20 ; Bull cMm,, 1902, pi, 27, 431. 5 Canada Pats. 209342, 214118 (1921) ; U.8. Pat 1334089 (1920). CHAPTER VII. COMPOUNDS OF NIOBIUM.

General. The compounds of niobium are not so numerous as those of vanadium. The 1 oxides, Nb O , NbO following 2 3 2, Nb 2O 5) are known, but 2 only the pentoxide gives rise to salts, viz. the niobates. The acid character of niobium pentoxide or "niobic acid" is very weak; the niobates are decomposed, for instance, by carbon dioxide, and are to the readily hydrolysed pentoxide. Niobic acid is, in fact, very com- in parable its method of preparation and behaviour to silicic acid and stannic acid. Reduction of pentavalent niobium compounds in acid solution with zinc yields solutions which appear to contain the niobium in the tetravalent state and probably also the trivalent state. The solution first becomes blue, then olive-green, and finally dark brown. Reduction of a dilute boiling, solution of niobic acid in concentrated hydrochloric or sulphuric acid may yield the brown solution immediately. The course of the reaction is affected considerably by such conditions as the acidity of the the solution, reducing agent employed, the physical condition of the reducing agent, and the presence of foreign substances. The final of the reduction can be stage usually depended on as being only approxi- trivalent mately (see p. 131). Electrolytic methods of reduction, using and 3 platinum amalgamated lead electrodes, have also been employed. The blue solutions gradually precipitate brown flakes, which are thereafter converted into white slowly niobic acid. Similar brown precipitates are obtained by the addition of ammonium hydroxide to the brown solutions. 4 Marignac concluded from the amount of potassium permanganate required to oxidise the brown precipitate that its composi-

tion was Nb O , which can be s 5 alternatively written NbO.2NbO 2 . This formula cannot, however, be definitely accepted, as the experimental data do not exclude the approximate empirical formula Nb 3O 4j, from which it would follow that the brown precipitate is composed of niobium sesquioxide together with small proportions of unreduced pentoxide. The blue solution functions as a powerful reducing agent, and will, for instance, precipitate copper from copper sulphate solution, and generally is a stronger reducing agent than trivalent titanium solution. On being evaporated in vacuo it leaves a damp blue mass, which on being dissolved in concentrated hydrochloric acid and treated with ammonium chloride 1 The existence of the oxide NbO is doubtful. 2 The natural ores called niobites consist of niobates. 8 Ber. Stabler, t 1914, 47, 841; Ott, Zeitech. Elektrockem.* 1912, 18, 349; Kiehl and Hart, /. Amer* Chem. Soc., 1928, 50, 1614. * Marignac, Ann. Chim. PKys.9 1866, [ir], 8, 15. 141 142 VANADIUM, NIOBIUM, AND TANTALUM. " a of niobium blue/* very similar in appearance to gives" precipitate molybdenum blue." l No definite salts have hitherto been isolated, however, from reduced niobium solutions, but an ammonium niobium sulphate, which has the probable formula (NHJaSO^M^SOJg.OHgO, and an acid ammonium niobium sulphate, (NH^SO^Nbg^O^.H^SO^ of niobium are devoid of 6H 2O, have been prepared. The halides saline character. than vanadic Niobic acid displays a much less pronounced tendency acid to form heteropoly-compounds with other acids, but oxaloniobates are known. It reacts with hydrogen peroxide to form perniobic acid, The double niobium HNb0 4.rH 20, salts of which are known. oxyfluorides also take up active oxygen.

NIOBIUM AND HYDROGEN.

The absorption of hydrogen by pure metallic niobium under different conditions of temperature and pressure has not been investigated. An impure sample absorbed 8 per cent, of hydrogen after fifteen hours' 2 3 exposure to the gas at a high temperature. In another experiment a substance of empirical formula NbH was obtained. This substance niobium NbF has also been prepared by fusing potassium fluoride, K 2 7 , with potassium fluoride (which has the effect of rendering th subsequent reaction less violent) and then reducing the product with sodium by heating strongly in a wrought-iron crucible. The excess of sodium is distilled off and the residue extracted repeatedly with water, and finally with water containing a small quantity of hydrofluoric acid. The 4 hydride is left behind as a black or dark grey powder. Its density 5 varies from 6-0 to 6*6. The specific heat of the hydride appears to an the decrease with increase of temperature ; impure sample gave value 0-0834 between and 440 C. 6 It resembles the metal in that it is insoluble in hydrochloric acid, nitric acid, dilute sulphuric acid, and aqua-regia, but it is attacked by concentrated sulphuric acid, hydrofluoric acid, and molten potassium hydrogen sulphate. On being heated in air it burns readily with incandescence to niobium pentoxide and water. It is scarcely affected by being heated in hydrogen, but it reacts with sulphur to form a black sulphide, and with chlorine and hydrochloric acid to form various niobium chlorides. Other niobium-hydrogen compounds or alloys of doubtful composition have been obtained by reduction of niobium oxytrichloride, 7 metallic niobium as the cathode NbOCl 3 , with hydrogen, and by using 8 in the electrolysis of dilute sulphuric acid.

Niobium and the Halogens. The halides and oxyhalides of niobium are set out in the following table:

1 See this series, Vol. VIL, Part III. (1926), p. 131. 2 Muthraann, Weiss, and Riedelbauch, Annalen, 1907, 355, 90. 8 von Bolton, Zeitsch. Ekktrockem., 1907, 13, H5. 4 Marignac, QompL rend., 1868, 66, 180. 5 Kruss and Nilson, Ojvers. K. Vet.-Akad. ForJi., 1887, No. 5 ; Ber. 9 1887, 20, 1691 ; compare Eoscoe, Chem. News, 1878, 37, 25. 6 Kruss and Nilson, toe. tit. 7 Blomstrand, Acta Univ. Lund, 1864 ; Boscoe, loc* cit. 8 von Bolton, loc. cit. COMPOUNDS OF NIOBIUM. 143

HALIDES AND OXYHALIDES OF NIOBIUM.

The pentavalent halides and oxyhalides, as in the case of other niobium compounds, are the most stable. It is remarkable that the pentavalency is maintained with increase in the atomic weight of the halogen. All the halogen compounds are characterised by their ready tendency to undergo hydrolysis on the addition of water or even in damp air with precipitation of niobic acid and formation of the hydrogen halide. Their preparation can, therefore, be effected only in the dry way: (a) synthetically, or (b) by the action of chlorine, carbon tetrachloride, or sulphur monochloride on the oxide or sulphide. They do not possess saline properties, and cannot be prepared by the action of the halogen acids on the oxide. Definite compounds of niobium and iodine are unknown, although tantalum pentiodide and vanadium tri-iodide have been prepared. The divalent chloro-compounds are probably more correctly represented as chloroniobium acid, HNb 3Cl7 .4.H 2O, and its derivatives, in Cl .4H the com- analogous manner to chlorotantalum acid, HTa3 7 2O, position of which has recently been reinvestigated.

NIOBIUM AND FLUORINE.

is the of Niobium Pentafluoride, NbF5 , only known compound niobium and fluorine, and even this cannot be obtained in the free state by a wet method because of the extreme readiness with which it hydrolyses. Niobium pentoxide dissolves readily in hydrofluoric acid, but evaporation of the solution leaves a residue of the unchanged oxide. x Niobium pentafluoride has been prepared synthetically by passing dry fluorine over the gently heated metal contained in a boat in a platinum tube. The product is freed from platinum tetrafluoride, a little of which is formed at the same time, by distillation in vacua at 100 to 110 C. An alternative method consists in treating niobium pentachloride with anhydrous hydrogen fluoride in a freezing mixture 2 purifying by redistillation. * Does not exist in the free state. 1 Ruff and Zedner, Ber. 9 1909, 42, 493. ? Ruff and Schiller, Zeitsch. anorg. Cfom., 1911, 72, 329. 144 VANADIUM, NIOBIUM, AND TANTALUM.

refractive ; Niobium pentafluoride forms colourless, highly prisms and boils at 217 to 220 C. density 3-2932 at 18 C. It melts at 75-5 C., It is under a pressure of 760 mm. of mercury. extremely hygroscopic at 236 C. and deliquesces rapidly in air. It is reduced by hydrogen Excess in contact with platinum to an unstable lower blue fluoride. attack of concentrated alkali hydroxide or alkali carbonate solutions it with formation of the alkali niobate. It dissolves in toluene, paraffin, carbon bisulphide, and other organic solvents. Niobium Double Fluorides of Niobium Pentafluoride. pentafluoride double fluorides with the shows a strong tendency to form stable the fluorides of other metals. These are conveniently prepared by action of carbonates of the metals on solutions of niobium pentoxide or the addition of a in a large excess of hydrofluoric acid, by large the of the metals. excess of hydrofluoric acid to solutions of oxyfluorides takes In the absence of excess of hydrofluoric acid hydrolysis place as usual with the formation of niobium oxytrifluoride, NbOF 3 . The indicates the existence precipitation of these double fluorides probable is the in solution of niobium pentafluoride ; stability imparted by atoms. formation of complex anions containing several fluorine co-ordination When viewed from the point of view of the Werner number seven is theory, it is observed that the co-ordination frequent in the double fluorides as well as in the niobium oxyhalides. Many and of the series members of the series 2R'F.NbF5 or [NbF7]R 2 ,

8 a metal and 2R'X.NbOX or R are known > where R represents 3 [~Nb o l six and X is fluorine, chlorine, or bromine. The co-ordination numbers thus : CsF.NbF or eight also occur, but less frequently, 5 [NbF6 ]Cs,

. in the co-ordination and 3NH F.NbOF or ) 3 The 4 3 rNb^l(NH 4 change number can be seen in the double ammonium oxyfluorides : NH 4F.

or 2NH F.NbOF or NbOF 3 Nb*(NH4 ) ; 4 3

6 all are known. NbOF or ) , of which 3 [NbQ l(NH 4 3 1 The following double fluorides of niobium have been prepared : Ammonium Niobium Fluoride. (See below under Potassium Niobium Fluoride.) . Cadmium Niobium Fluoride, 3NbF5.5CdF 2.5HF.28H 2O or Nb 3Cd5F 26 the action of 5HF.28H 2O, is obtained in long, transparent prisms by cadmium carbonate on a solution of niobic acid in concentrated hydrofluoric acid. It is insoluble in water. or is obtained in fine Ccesium Niobium Fluoride, CsF.NbF5 CsNbF^ needles by repeated crystallisation of caesium niobium oxyfluoride, acid.2 Another caesium niobium fluoride Cs 2NbOF5> from hydrofluoric or Cs NbF has been having the probable composition 7CsF.NbFs 7 s prepared by the action of a solution of caesium hydroxide in hydro- 3 fluoric acid on niobic acid in the same solvent. Cobalt Niobium Fluoride, 8NbFs.5CoF 2.5HF.28H 2 or Nb 3Co 5F 25 .

1 68 Bull. Marignac, Ann, Chim. Phys.9 1866, [iv], 8, 19, ; Santesson* Soc^ 1875, [ii], 24, 38, 67. a Balke and Smith, /. Amer. Chem. Soc., 1908, 30, 1637. 3 Pennington, ibid., 1896, 18, 59- COMPOUNDS OF NIOBIUM. 145 5HF.28H forms dark 20, red, prismatic crystals, which are prepared similarly to the corresponding cadmium salt. Niobium Copper Fluoride, NbF6.2CuF 2.HF.9H 2O or NbCuF .HF. 9H forms dark 9 2O, large, blue crystals, which are obtained similarly to the cadmium salt. Ferrous Niobium Fluoride, 2NbF .3FeF .4HF.19H or 5 2 2O Nb aFe 3F16 . 4HF.19H 20, is obtained in thin greenish-yellow, prisms by dissolving iron and niobic acid in equivalent proportions in hydrofluoric acid. Niobium Manganese Fluoride, 3NbF5.5MnF 2.5HF.28H O or Nb Mn F .28H 2 3 5 25 20, forms rose-coloured, long prisms, which are obtained similarly to the cadmium salt. Niobium Mercury Fluoride, NbF5 2.8H or F .3HgF 2 NbHg 3 1]L .8H 20, is obtained mercuric by dissolving oxide and niobic acid in hydrofluoric acid. Concentration of the solution after removal of mercuric fluoride a white yields mass which consists of aggregates of short prismatic needles. Nickel Niobium 2NbF Fluoride, .3NiF .4HF.19H or . 5 2 2O Nb 2Ni 3F16 4HF.19H O, forms needles 2 thin, green which are prepared by dissolving of equivalent quantities niobic acid and nickel carbonate in hydrofluoric acid. Concentration of the mother-liquor gives dark green, flat of the 3NbF .5NiF prisms composition 5 2.5HF.28H 2O or NbJSfi3 sJF!25 0. 5HF.2SH 2 . Potassium Niobium NbF .2KF or is Fluoride, 5 K 2NbF7 , obtained by a cooling concentrated solution of potassium niobium oxyfluoride NbOF .2KF.H or 3 2 K 2NbOFs.H 2O, in hydrofluoric acid. It forms

small, glistening, rhombic needles, a : b : c=0-6682 : 1 : 0-4699. It is with the isomorphous corresponding tantalum salt but not with potassium titanium 1 fluoride, K 2TiF 6 , and hence is used in one method for the of titanium from separation niobium and tantalum (see p. 128). It is stable at 100 C.*, but loses hydrogen fluoride at considerably higher temperatures. In damp air it is slowly converted into the oxyfluoride, K NbOF . Its solution 2 5 in hot water precipitates the hydrated oxy- 2 fluoride, K 2NbOF5 .H 2O, and leaves an acid solution. The corresponding sodium niobium fluoride is unknown. Am- monium niobium fluoride cannot be prepared in the pure state because hydrolysis takes place even in the presence of excess of hydrofluoric acid. A solution of ammonium niobium oxyfluoride, 3NH4F.NbOF3, in acid the salt hydrofluoric precipitates double 2NH 4F.NbFs .NH4F. NbOF or in 3 (NH 4 ) 3NbF 8 .NbOF 3 masses of small prisms. Rubidium Niobium or is Fluoride, RbNbF6 RbF.jSCbF5 , obtained by of repeated crystallisation rubidium niobium oxyfluoride, Rb 2NbOF5, from 3 hydrofluoric acid. The double salt Rb 2NbF 7 or 2RbF.NbF5 has 4 also been reported. Zinc Niobium Fluoride, 3NbF5.5ZnF 2.5HF.28H 2O or Nb 3Zn5F25. 5HF.28H20, is prepared in long prisms by dissolving zinc and an equivalent quantity of niobic acid in hydrofluoric acid. Oxyfluorides* Two oxyfluorides of niobium having the compositions NbOF 3 and Nb0 2F are known. The former has been obtained in small crystals by the action of hydrogen chloride on a fused mixture

1 /. HaU, Amer. Chem. Soc. t 1904, 26, 1240. 2 Buff and Schiller, Zoa oft. ; Mefczener, Ber. f 1913, 46, 983. 3 Balke and Smith, foe. tit. * Pexmington, Zoc. tit. VOL. VI. : III. 1Q 146 YANADIUM, NIOBIUM, AOT) TANTALUM. at a red heat.1 The existence of niobium pentoxide and calcium fluoride with of the latter has been recognised only in double salts potassium in the free state. fluoride ; it has not been isolated are obtained the Double salts of niobium oxytrifluoride, NbOF 3, by in action of fluorides of the metals on solutions of niobium pentoxide latter avoided, otherwise the hydrofluoric acid, excess of the being are much more stable double fluorides are produced. These salts than the double fluorides of niobium and other metals, and are, in fact, The among the most stable of the pentavalent niobium compounds. the double oxyfluorides which are most readily prepared possess general stands for a monovalent metal. formula 2RT.NbOF 3.a?H 20, where IT other fre- Recrystallisation of double oxyfluorides having compositions facts indicate the existence of quently yields salts of this type. These the a stable, divalent, complex anion of constitution [NbOFJ", stability that is in of which is confirmed by the great difficulty experienced of niobium the electrolytic reduction of solutions potassium oxyfluoride, 2 It is of interest to note that double oxyfluorides K 2NbOF5 .H 20. of vanadium, tantalum, and molybdenum having similar compositions react with are also known. Double oxyfluorides of the alkali metals which are hydrogen peroxide to yield alkali niobium peroxyfluorides, described on p. 168. or have The following double oxyfluorides of niobium, fluorozyniobates, 3 been prepared : are known : Ammonium Niobium Oxyfluorides. The following F.3NbOF .H or NH 4F.NbOF3 or NH4NbOF 4 ; 5NH4 3 2 (NH 4 ) 5: or or NbOF ; 3NH F.NbOF 3 Nb,0 3F14.H 2 ; 2NH4F.NbOF 3 (NH 4 ) 2 5 4 is the com- . Of the 2NH F.NbOF 3 (NH 4 ) 3NbOF6 these, compound 4 fluoride monest, and is readily prepared by the action of ammonium bn a solution of niobium pentoxide in hydrofluoric acid. It behaves but is not similarly to the corresponding potassium salt (see next page), isomorphous with it. It is isomorphous with ammonium tungsten and forms rhombic in which oxyfluoride, 2NH 4F.WO 2F 2, bipyramids loss a : b : c=0*4184 : 1 : 1-0058. It can be heated to 170 C. without

5 co-ordinated formula is . in weight. Its |Nb (NH 4 ) 2 By using excess of niobium pentoxide in the last preparation the is obtained. This is also compound 5NH4F.3NbOF 3 .H 2O very comparable in its behaviour to the corresponding potassium salt. The mother- of F.NbOF or 4 NH liquor yields green crystals composition NH4 3 NbQ 4 on being evaporated. If excess of ammonium fluoride is employed, of the 3NH F.NbOF or 6 are crystals compound 4 3 fNbQ l(NH4 ) 3 pro- 4 duced. All these double ammonium oxyfluorides can be heated to 100 C. without undergoing decomposition. Ccesium Niobium Oosyfluoride, 2Cs.F.NbOF3 or Cs 2NbOF6 or 5 Cs is to the salt NbQ 2 , prepared similarly corresponding potassium (see below). It forms trigonal crystals.

1 2 Joly, Compt, rend., 1875, 8r, 1266. Ott, Dissertation (Munich), 1911, 51. 3 Ann. Ghim. Marignac, Phys., 1866, [iv], 8, 19, 68 ; Gompt. rend., 1868, 66, 180 ; J. Amer. Ghem. 67 Balke Pennington, Soc., 1896, 18, 38, ; ajid Smith, ibid., 1908, 30, 1637. 4 Baker, J. Chem. Soc., 1879, 35, 760. COMPOUNDS OF NIOBIUM. 147 Niobium Copper Ocvyfluoride, CuF a .NbOF 3.4H 2O or CuNbOF5 .

or 5 is 4H 2O Nb 2O, prepared by the action of copper fluoride I Q |Cu.4H on a solution of niobium pentoxide in hydrofluoric acid. It yields blue, monoclinic prisms which are very readily soluble in water and which are stable at 100 C. They are isomorphous with copper titanium .4H and with fluoride, CuTiF 6 20, copper tungsten oxyfluoride, CuW0 2F4 . 4H 2O.

Potassium Niobium Oxyfluorides. The following are known : 4KF. or 3NbOF 3.2H 2 K 4Nb 3 3F13.2H 2 ; 5KF.3NbOF 3 .H 2 or F .H .H or K 5Nb3O s 14 2 ; 2KF.NbOF 3 2 K 2NbOF5.H 2 ; 8KF.NbOF 8

. these or K 3NbOF 6 Of potassium compounds the salt 2KF.NbOF 3 . H or 5 K .H Q is the most and in one of 2 NbQ 2 2 important, is, fact, the commonest of niobium compounds, as it is the usual end product in the working of niobium minerals, and is the form in which niobium is separated from tantalum by the classical method due to Marignac (see p. 128). It is prepared by the addition of potassium fluoride to a hydrofluoric acid solution of niobium pentoxide, or by recrystallisation of other potassium niobium oxyfluorides or of potassium niobium fluoride, from water. It K 3NbF7 , undergoes recrystallisation unchanged in very thin leaflets from pure water. In the presence of a slight excess of hydrofluoric acid it forms thin, monoclinic scales which are isomorphous with potassium fluoroxytungstate, 2KF.W0 2F 2 .H 20,, and with potassium titanium fluoride, 2KF.TiF 4.H 20. Its crystal elements are : a:b:c

: : 46'. =0-992 1 0-980 ; j8=103 It loses almost all its water at 100 C., but undergoes no further change and remains completely soluble in water when heated to 200 C. One part of the salt by weight dissolves in from 12-5 to 13 parts of water between 17 and 21 C. It is much more soluble in hot water and in water containing hydrogen fluoride or hydrogen peroxide, and is also much more soluble than the potassium tantalum oxyfluorides. The solution is precipitated by the addition of caustic caustic the is soluble soda or potash ; precipitate in a slight excess of the alkali, but separates out again in crystalline form when large excess is added. In this respect the compound behaves differently from potassium titanium fluoride, but the reaction is not, however, available for the separation of niobium and titanium, as, when both metals are x present, the precipitate also contains both. Hall and Smith have made a study of the differential action of seventy-four bases, both inorganic and organic, on solutions of potassium niobium oxyfluoride and potassium titanium fluoride. or .H is The compound 5KF.3NbOF 3.H 2 K5Nb 3 3F14 2 obtained from the mother-liquor in the preparation of the above salt when the potassium fluoride has been added only sparingly to the solution of niobic acid in hydrofluoric acid. The new mother-liquor thus obtained

or . yields crystals of the composition 4KF.3NbOF3.2H 2O K4Nb 3 3F13 2H 2O. The former of these gives, apparently, hexagonal prisms* They at C. this lose only part of their water 100 ; above temperature hydrogen fluoride is evolved. When the quantity of potassium fluoride added to the solution of niobic acid in hydrofluoric acid is somewhat greater than that required for the formation of the salt 2KF.NbOF 3.H2O, crystals of a salt having 1 Hall and Smith, Ohem. New, 1905, 92, 253. 148 VANADIUM, NIOBIUM, AND TANTALUM, or NbOF are obtained. This com- the composition SKF.NbOF s K 3 6 acid with excess ot pound has also been prepared by fusing niobic 1 at 100 C. Its solution m potassium hydrogen fluoride. It is stable JI so that the change hot water yields crystals of 2KF.NbOF 3 20,

2KF.NbOF3.H 20+KF^3KF.NbOF3 +H 2

of 8KF.NbOF . appears to be reversible. A hydrofluoride composition 3 HF has been obtained from solutions containing large excess of hydro- Slow from con- fluoric acid or of potassium fluoride. crystallisation s beautiful c* ' centrated solutions gives rise to very prismatic ys^ aKt.bnl Lb ; isomorphous with the double potassium tin fluoride, 4 14'. This substance is also a b c=0-62T9 : 1 : 0*4900; =93 quite loses fluoride on being very strongly stable at 100 C. s and hydrogen only

.H is boiled for twelve ^If a solution of the compound 2KF.NbOF 3 2 while steam is introduced, a white, soft, micro-crystalline powder hours 2 of which is 2KF.3Nb0 F or K 2Nb 3 6F5 . is obtained, the composition 2 has not hitherto been isolated. A compound of the formula Nb0 2F .3H and 3KF.2Nb .5H are of The compounds KF.Nb 2 5 2 s 5 2 doubtful constitution. They are obtained as crystalline powders by with fluoride and the fusing niobium pentoxide potassium extracting 3 melt with boiling water. or NbOF or Rubidium Niobium Oxyfluoride, 2RbF.NbOF 3 Rb 2 5

is to the potassium a, prepared similarly corresponding compound. so well defined as Sodium Niobium Oxyfluorides. These are not of rise to a the potassium salts. Similar methods preparation give T which associated precipitate of 2NaFJX bOF 3.2H 2O, is, however, always difficult to with some of the salt NaRNbOF 3.H 20, and the two are 3NaF.NbOF however, been obtained separate. The compound 3 has, NaCaNb F, is also pure. A sodium-calcium-niobium oxyfluoride, 2 6 known.4 It is obtained by fusing lime, niobium pentoxide and sodium of 4-196 to fluoride together. It forms colourless octahedra density 4-341, these and its refractive index for sodium light is 2-148 to 2-150 ; figures agree with those given by natural pyrochlore (see p. 121). or Tl or Thallium Niobium Oxyfluoride, 2TlF.NbOF3 2NbOF5

is obtained in colourless, transparent, rhombic crystals by the action in solution of thallium .fluoride on niobium pentafluoride. or .6H Zinc Niobium Oxyfluoride, ZnF 2.NbOF 3.6H 2 ZnNbOF5 2 or 5lZn.6H results from the action of zinc fluoride on a solution rNbQ 20, of niobic acid in hydrofluoric acid. It gives rise to rhombic, hexagonal zinc titanium crystals, which are isomorphous with those given by fluoride, ZnF2TiF4.6H20.

1 Baker, J. Ghem* Soc., 1879, 35, 761. 2 KrGss and Nilson, tyvers. K. Vet^AJcad. Fork., 1887, No. 5; Ber., 1887, 20, 1689. * Petersen, /. prakt. Chem., 1889, [ill, 4<>> 287. * Holmquist, Zeitech. Kryst. Hin., 1899, 31, $06. COMPOUNDS OF NIOBIUM. 149

NIOBIUM AND CHLORINE. " Chloride or niobium . Ghloroniobium dichloride," Nb 6Cl14 .7H O. 1 This is 7H 2O or [Nbc Cl12]Cl 2 2 compound prepared by heating 1 part of niobium pentachloride with 7 parts of 3 per cent, sodium amalgam in a hard-glass combustion tube from which the air has previously been exhausted. The product is allowed to cool in a vacuum, is extracted with hot water, filtered from a brown oxide formed at the same time, and then evaporated with the addition of hydrochloric acid. It forms black, shining crystals, which give an olive-green powder, soluble in hot water. It is only slowly decomposed by ammonium nitric acid hydroxide ; concentrated boiling precipitates niobic acid. Only two of the chlorine atoms are ionised in solution. On being treated with the equivalent quantity of caustic soda it yields chloroniobium hydroxide, [Nb 6Cl12](OH) 2.8H 20, as a black, micro-crystalline precipitate, which in turn forms chloroniobium bromide, Nb6Cl12Br 2 . 7H 2O, when evaporated with hydrobromic acid. Chloroniobium chloride and chloroniobium hydroxide are both soluble in excess of caustic soda, from which solution the addition of excess of concentrated hydrochloric acid precipitates a brown powder having the composition is less Nb6 Cl14.9H 20. This substance soluble than the olive-green compound into which it is converted by boiling water. The available evidence suggests that the brown compound differs constitutionally 2 their behaviour recalls from the green compound ; the different forms in which chromium trichloride is known to exist. 3 NbCl is Niobium Trichloride, 3, prepared by leading the vapour 4 of niobium pentachloride through a heated tube. It is also formed in small quantity by the action of carbon tetrachloride vapour on niobium 5 pentoxide contained in a hard-glass tube, and has probably been 6 prepared in solution by the electrolytic reduction of the pentachloride. It forms a black, crystalline crust with an almost metallic lustre, which closely resembles the appearance of a film of sublimed iodine. It is not decomposed by water or ammonia, but is readily oxidised by dilute nitric acid to niobium pentoxide. On being heated to a red heat in an atmosphere of carbon dioxide, a sublimate of niobium oxytrichloride, is the carbon dioxide reduction to the NbOCl 3 , produced, undergoing monoxide. Niobium Tetrachloride is reported to have been obtained in solution by the electrolytic reduction of a solution of sodium niobate 7 in hydrochloric acid. is the of Niobium Pentachloride, NbCl5 , most important the chlorides of niobium, and is the material from which the other chlorides are prepared. It can be obtained by several methods :

1 It should be noted that the formula HTa3Cl 7 .4H 2 has been recently ascribed to the corresponding tantalum chloride, as well as similar formulae to analogous compounds of molybdenum and tungsten. The forrnulao here given for niobium dichloride and its derivatives are, therefore, perhaps incorrect. Earned, J. Amer. Ckem. Soc., 1913, 35, 1078. See this series, Vol. VII., Part III. (1926), p. 24. Roscoe, Chem. News, 1878, 37, 26. Belafontaine and Linebarger, J. Amer. Chem. Soc., 1896, 18, 532. er. 842. Ott, Dissertation (Munich, 1911) ; compare Stabler, 9 1914, 47, 7 Ott, loc. cit. 150 VANADIUM, NIOBIUM, AND TANTALUM.

1 (a) By heating metallic niobium in chlorine at a red heat. (b) A mixture of niobium pentoxide and carbon is gently heated in a stream of chlorine, air being excluded. The pentachloride is sublimed away from any oxychloride simultaneously produced by 2 raising the temperature at the conclusion of the reaction. (c) By the action of the vapour of carbon tetrachloride on niobium 3 pentoxide heated to 230 to 250 C. Some of the oxychloride is formed at the same time, but the change to the pentachloride is complete if the 4 heating is carried out in a sealed tube at 200 to 225 C. The same change takes place if the niobium pentoxide is subjected to the action 5 of a mixture of chlorine and carbon tetrachloride vapour, or chlorine 6 7 and sulphur dichloride, or phosphorus pentachloride, or sulphur 8 monochloride. In the last case the pentachloride is removed from excess of sulphur monochloride by distillation. (d) By carefully heating niobium sulphide in chlorine free from 9 oxygen. Niobium pentachloride crystallises in long, yellow, transparent 10 needles. Density 2*78 to 277. The specific electrical conductivity 6 of the fused substance between 220 and 235 C. is 0-22 xlO~ reciprocal ohms (the corresponding figure for copper at ordinary temperatures 4 is is insulator of the 64X10 ), so that fused niobium pentachloride an 11 order of the best conductivity water. It melts at 212 C. to a red 12 liquid, which volatilises readily and boils at 240-5 C. Its vapour is yellow. Its density between 280 and 300 C.=9-45 (air=l. NbCl5 requires 9'35). The chloride sublimes unchanged in the vapour of carbon bisulphide, and dissolves readily in sulphur monochloride to a red solution and in carbon tetrachloride to a yellow solution; both these solvents yield crystals on being concentrated. It is also soluble in chloroform, alcohol, and ether ; a nitride of niobium is precipitated when ammonia is passed into the ether solution. Niobium pentachloride on to air undergoes ready hydrolysis ; being exposed damp it evolves hydrochloric acid fumes and becomes coated with a skin of niobic acid. It is completely decomposed by water to hydrochloric acid and niobic acid, which latter can be obtained pure by repeated washing with water. It dissolves in concentrated hydrochloric acid to produce a clear solution, and in concentrated sulphuric acid with evolution of hydrochloric acid gas. Double chlorides of niobium pentachloride with chlorides of other metals to correspond with the double fluorides given by niobium pentafluoride are not known. double . A compound with piperidine, NbCl5 1 von Zeitsch. Bolton, Ekktrochem., 1907, 13, 147 ; compare Moissan, Gompt. rend., Soc. 1901, 133, 20 ; Bull chim., 1902, [iiij 27, 431. 2 Anwlen, 433 ; Rose and Rose, Pogg. 1858, 104, Weber, ibid., 1853, 90, 462 ; compare Biltz and Voigt, Zeitsch. anorg. Ghem., 1921, 120, 71. 3 Funk and Niederlander, Ber., 1928, 61 B, 249, 1385; Demarcay, Gompt. rend., 1887, 104, 111 ; compare Camboulives,t"6, 1910, 150, 175. 4 Hall and Ghem. Smith, News, 1905, 92, 277 ; Ruff and Thomas, Zeitsch. anorg. Chem., 1926, 156, 213. 5 Ruff and Schiller, ibid., 1911, 72, 329. 6 Bourion, Ann. Ghim. Phys., 1910, [viii], 20, 565. 7 Pennington, /. Amer. Ghem. Soc., 1896, 18, 61. 8 Smith, ibid., 1898, 20, 292 ; Hall and Smith, loc. cit. * Biltz and Gonder, Ber., 1907, 40, 4969 ; Biltz and Voigt, kc. tit. 10 Balke and Smith, J. Amer. Ghem. Soc., 1908, 30, 1637. 11 Biltz and Voigt, loc. cit. 12 Deville and Troost, Gompt. rend'., 1865, 60, 1221 ; 1867, 64, 294. COMPOUNDS OF NIOBIUM. 151

^N, has been prepared, however, by addition of the organic base 1 to a concentrated alcoholic solution of the pentachloride. It forms white needles which undergo decomposition on attempts to recrystallise. 2 Similar compounds are formed with aniline, pyridine, etc. Niobium Oxychlorides. The most important niobium oxy- NbOCl which is chlorine compound is niobium oxytrichloride, 3, described below. of niobium in By the electrolytic reduction of a solution pentoxide acid and dilute (a) concentrated hydrochloric (&) hydrochloric acid, carbon and subsequent evaporation in vacw or in an atmosphere of been dioxide, two compounds having the following compositions have 3 reported :

Cl or . (a) Nb 2(OH) 2 4.5H 2 NbOCL[Nb(OH 2 )6 ]Cl 3 ei .3H or Cl (b) Nb 2(OH) 3 3 2 NbOCL[Nb(OH 2 ) 4 2]. has been mentioned ChloronioMum hydroxide, [Nb6 CI12](OH) 2.8H 20, in describing chloroniobium chloride (see p. 149). . is of historical Niobium Oxytrichloride, NbOCl 3 This compound interest in that it was one of the oxy-halogen compounds prepared Rose in which the atom was overlooked. Rose originally by " oxygen " chloride its correct called this substance hyponiobium ; formula, and hence the true valency of niobium, were first established by Blomstrand.4 a Niobium Oxytrichloride is prepared by the action of chlorine on or the lower Nb0 6 mixture of niobium pentoxide and carbon, on oxide, 2 , 6 or by the action of the vapours of carbon tetrachloride on the pentoxide. and this Some niobium pentachloride is produced in the same reaction, is removed either by distilling it away at the lowest temperature possible in a in an atmosphere of carbon dioxide, or by subliming the product 7 has also current of chlorine over the ignited oxide. The oxyehloride niobium over been prepared by passing the vapours of pentachloride 8 niobium pentoxide at a red heat. which sublime Niobium Oxytrichloride forms white, silky crystals, in a sealed tube. It is to silky needles on being heated very stable. It can be volatilised at 400 C. without melting; its vapour 860 C. 7*87 * NbOCl density at 440 C. is 7-89, and at (air=l. 3 is colourless. On heated in requires 7-47). Its vapour being gently it sublimes ; with hydrogen, carbon dioxide, or chlorine, unchanged carbon it de- very strong heating in hydrogen or dioxide, however, and niobium composes and forms niobium pentoxide pentachloride It is soluble in (hydrogen also produces some of the lower oxides). alcohol and ether. Like the pentachloride it undergoes ready hydrolysis acid it is attacked with water, producing niobic acid and hydrochloric ; by damp air. It dissolves in alkalis, also in concentrated sulphuric

1 Benz, Zeitsch. anorg. Chem., 1903, 36, 103. 8 loc. cit. Hall, loc. tit. ; Tuak and Niederlander, 3 Qto, Dissertation (Munich; 1911). * Blomstrand, Annalen, 1865, 135, 168. * Devffle and Troost, Compt. rend., 1865, 60, 1221. /. w Ufiem. 6 Delafontaine and Linebarger, /. Amer. Ohem. 8oc., 1896, 18, 532 ; Hews, of nio"blum 1896, 74, 33 ; compare preparation pentachloride. Hall and Smith, loc. cit. 6 Deville and Troost, Cowpt. rend,, 1867, 64, 294. * Devilie and Troost, loc. cit. 152 VANADIUM, NIOBIUM, AND TANTALtM. of acid acid, to form a clear liquid with evolution hydrochloric gas> concentrated acid. but it is only very sparingly soluble in hydrochloric Niobium Double Sails of Niobium Oxytricldorite. oxytnchlonde with alkali chlorides and with the gives rise to two series of double salts These have the formulae organic bases pyridine and quinoline. general are either by addition NbOCl 3.RCl and NbOCl s.2RCL They prepared of niobic acid in of the alkali chloride or organic base to a solution the 'action of the alkali chloride concentrated hydrochloric acid, or by in alcohol solution. or organic base on niobium oxytrichloride They air but are water. crystallise well, are stable in dry decomposed by The following are known :

. unstable. NbOCl 3.2NH 4Cl . Very

. octahedra. NbOCl 3.2RbCl . . Yellow, regular Do. do. do. NbOCl s.2CsCl ... . . Pale NbOCl 3.C5H5N.HCl greenish-yellow crystals. Colourless NbOCl 3 .C5H6N.HCl.HoO crystals. . NbOCl 3.2C5H6N.HCLH 2 Large, colourless, transparent prisms. or , . Almost colourless needles greenish- NbOCl 3.C9H7N.HCl yellow truncated prisms.

. . Almost colourless needles. NbOCl 3.2C9H7N.HCl but The corresponding bromine compounds are very comparable, were unsuccessful attempts to prepare corresponding iodo-derivatives acid. Double owing to the insolubility of niobic acid in hydriodic tantalum compounds of vanadium oxytrichloride with pyridine, and also been oxytrichloride with pyridine and quinoline, have prepared.

NIOBIUM AND BROMINE.

. is bromine Niobium Pentabromide, NbBr5 This the only the action compound of niobium hitherto prepared. It is obtained by 2 a mixture of bromine vapour on the coarsely powdered metal, or on 3 the latter of niobium pentoxide and carbon in the absence of air. In case some of the oxybromide is also formed. Niobium pentabromide in is a fine, purple-red powder, very similar to red phosphorus appearance. The fused substance forms garnet-red prisms. On being strongly heated it becomes yellow, and volatilises. It melts at about . at about 150 (X, and distils undecomposed in an inert atmosphere 270 C. It can be distilled unchanged in an atmosphere of nitrogen or carbon dioxide. It is very hygroscopic, hydrolyses rapidly in damp considerable air, and is decomposed by water with a hissing noise and evolution of heat into niobic acid and hydrobromic acid. It is soluble in absolute alcohol and in dry ethyl bromide. is the action of Niobium Oxytribromide, NbOBr 3 , prepared by bromine on a mixture of niobium pentoxide and carbon. The method is that by which the pentabromide is produced, but in this case the proportion of carbon present is much smaller. The product is distilled 4 in an atmosphere of bromine or nitrogen. It is a bright yellow, voluminous substance, which sublimes without melting in an atmosphere of bromine, but is converted completely into niobium pentoxide on being sublimed in an atmosphere of carbon dioxide. It fumes in damp 1 Weinland and Storz, Ber., 1906, 39, 3056 ; Zeitech. anorg. Chem., 1907, 54, 223. 2 Barr, J. Amer. Ohem. &oc., 1908, 30, 1668. 3 4 Rose, Poffg. Annakn, 1858, 104, 442. Ban, lac. cit. COMPOU3STDS OF NIOBIUM. 153

air and is decomposed by water with formation of niobic acid and hydrobromic acid. It is soluble in hot concentrated sulphuric and hydrochloric acids, in absolute alcohol and dry ethyl bromide. Double Salts of Niobium Oxytribromide. These correspond exactly to the double salts of niobium oxytrichloride described above, and are * prepared by analogous methods. The following are known :

NbOBr 3 .2RbBr . . . Small, dark red octahedra. NbOBr 3.2CsBr . . . . Pale yellow octahedra.

NbOBr 3.C5H5N.HBr . . Orange-red crystals. NbOBr3.CsH7N.HBr . . Do. prisms. These double salts are even more unstable in damp air than the corresponding chlorine compounds. Chloroniobium bromide, [Nb6Cl12]Br 2.7H 20, has been mentioned in describing the properties of chloroniobiurn chloride (see p. 149).

NIOBIUM AND IODINE.

Definite compounds of niobium and iodine are unknown. The 2 preparation of an iodide from the pentabromide has been reported but no details are supplied. A pyridine addition compound of the has been 5 its pentiodide, NbI 5 .(C5H 5N.HI) 6 , described, but existence lacks confirmation.4 NIOBIUM AND OXYGEN. Niobium Monoxide, NbO, is of considerable interest as a compound of divalent niobium, but its existence is doubtful. Attempts to prepare it by reduction of the pentoxide with hydrogen at high temperatures and in the carbon electric vacuum furnace 5 have not been successful. It is stated to be with the formed together dioxide, Nb0 2, by treating the pentoxide with hydrogen at about 150 atmospheres pressure and 2500 C. 6 Rose 7 obtained it by reducing potassium niobium oxyfluoride, K 2NbOF5> with sodium in an iron crucible and then removing the fluorides of sodium and potassium in the product with water. Both 8 Rose and Hermann overlooked the presence of the oxygen atom, and looked upon this substance as being metallic niobium. A study of the amount of oxygen absorbed in its conversion into the pentoxide showed, 9 however, that it was more probably the oxide NbO. This conclusion is supported by the following facts : (1) Niobium monoxide is stated to also or be prepared by reducing niobium oxytrichloride, NbOQ 3, 10 it is converted niobium oxytrifluoride, NbOF $, with magnesium; (2) into niobium oxytrichloride, NbOCl 33 on being heated in a stream of chlorine,

2NbO+3Cl 2 ==2NbOCl 3 ;

1 Weinland and Storz, kc. c#. 2 Barr, loc. cit. 3 Renz, Zeitech. anorg. Chem., 1903, 36, 106* 4 Weinland and Storz, loc. cit. 5 Ruff, Seiferheld, and Suda, Zeitsch. anorg. Chem., 1913, 82, 373 ; compare Haiignac, Oompt. rend., 1868, 66, 180. 6 Newbery and Pring, Proc. Roy. Soc., 1916, [A], 92, 276. 7 Rose, Pogg. Annalen, 1858, 104, 310. 8 Hermann, J. prakL Chem., 1871, 3, 386. 9 Delafontaine, Arch. Sci. $hys. not., 1866, [ii], 27, 167. 10 Deville and Troost, Compt. rend., 1865, 60, 1221. 154 VANADIUM, NIOBIUM, AND TANTALUM. on dissolved in (3) it is converted into niobium oxytrichloride being hydrochloric acid, 2NbO+6Ha=2NbOCl 8 +8H 1.

The monoxide has been variously described, according to the method or or beautiful of preparation, as a black powder, glistening crystals, it is a con- black cubes. Its density varies from 6-3 to 6-7, and good in air it oxidises to the ductor of electricity. On being heated readily of heat. It dissolves in pentoxide with considerable evolution hydro- of and a fluoric acid and hydrochloric acid withevolution hydrogen, solution. It is also dissolved pentavalent niobium salt is formed in by to form niobate. This behaviour boiling potassium hydroxide potassium of niolnum monoxide towards acids and alkali indicates that divalent of salts of niobium are too unstable to exist. A divalent chloride isolated tantalum has, however, recently been (see p. 192). has been as a bluish- Niobium Sesquioxide, Nb 2 3, prepared (a) with ; black powder by reducing niobium pentoxide magnesium powder are removed the magnesium oxide so formed and excess of magnesium acid l as a with a with dilute hydrochloric ; (b) bright grey powder the in a stream of at bluish tinge by heating pentoxide hydrogen 1250 C. The latter method yields a product which is free from "both 2 melts at 1780 C. Its metal and higher oxides. The pure product 3 with electrical conductivity has been measured. On being mixed carbon and heated in hydrogen at 1200 C. it yields niobium carbide,

. this oxide is Niobium Dioxide, NbO 2 Like the sesquioxide, obtained by reducing the pentoxide with hydrogen at a high temperature.4 It has also been prepared by the electrolysis of fused potassium in a vessel, which acts as niobium oxyfluoride, K 2NbOF5 , platinum 5 the cathode, and using an anode of the same material. Niobium at dioxide is a black powder which is stable in air ordinary tempera- heated in air it forms the white It is not tures ; on being pentoxide. attacked by hot or cold acids, including hydrofluoric acid and aqua- solutions dissolve it slowly. Its regia ; boiling potassium hydroxide studied Goldschmidt.6 crystal structure has been by is the Niobium Pentoxide, Niobic Anhydride, Nb 2 5 , probably commonest compound of niobium. It is obtained in the treatment of is the material in niobium-bearing minerals (see p. 124), and starting It is in the the preparation of other niobium compounds. precipitated all niobium hydrated state by the hydrolysis of nearly pentavalent estimation of niobium 130 salts, and is formed in the gravimetric (see p. ). It can be prepared by heating metallic niobium or any of the lower air or oxides, or the sulphide, carbide, or nitride of niobium, in oxygen. also the Ammonium niobium oxyfluoride, (NH 4 ) 2NbOF5 , yields pent- a oxide on being heated in air. Solutions of the alkali niobates yield when treated with mineral white gel of the hydrated pentoxide acids, sulphuric acid being usually preferred. Insoluble niobates on fusion

1 Smith and Maas, Zeitsch. anorg. Chem., 1894, 7, 98. 2 169 127. Friederich and Sittig, ibid., 1925, 143, 293 ; 1925, 144, ; 1925, 145, 3 Friederich, Zeitsch. Physik, 1925, 31, 813. * Zeitsch. 213 Delafontaine, toe. cit. ; Ruff and Thomas, anory. Chem., 1926, 156, ; compare Kewbery and Pring, loc. cit. 5 Ott, Dissertation (Munich, 1911). 6 Goldschmidt and others, Amer. Chem., Abs., 1926, 20, 3414. COMPOUNDS OF NIOBIUM. 155

with potassium hydrogen sulphate, and extraction with water, yield a residue of niobic acid, which is repeatedly washed with water containing a little ammonia to remove the adhering sulphuric acid. The gel is then ignited, preferably with the addition of a small quantity of ammonium carbonate, and a residue of pure, dry niobium pentoxide is obtained. The pentafluoride, oxyfluorides, and double alkali fluorides similarly yield the pentoxide if they are evaporated with concentrated sulphuric all fluoride has acid until the hydrogen been evolved ; the residue is carefully washed and ignited. It is a difficult matter to wash precipitated niobic acid free from adsorbed mineral acids because of its colloidal character. Traces of alkali, which may also be present in the product, are best avoided by starting with the ammonium salt. Properties. Anhydrous niobium pentoxide is a white, amorphous, 1 tasteless, odourless powder which remains white on being heated, 2 although some samples assume a temporary yellow colour while hot. It is a remarkably stable substance and can be heated to 1750 C. 8 without undergoing decomposition. When precipitated niobium pentoxide is heated to dull redness it glows brightly for a short time, the extent of the glow and the temperature at which it occurs depending on the method of preparation. Various explanations of this have been

: size put forward (a) Allotropic transformation ; (b) increase in of transition to state. particles ; (c) from amorphous crystalline Recently, Debye-Scherrer X-ray spectrograms, made on samples of the oxide before and after glowings, have shown that transition from the amorphous to a crystalline state takes place. The same explanation is found to hold good for the glowing of tantalum pentoxide, ferric oxide, 4 chromium sesquioxide, titanium dioxide, and other oxides, and is consistent with the fact that niobium pentoxide assumes a crystalline 5 form on being very strongly heated. Fusion with borax also yields 6 7 green, rhombic prisms. Microscopic, biaxial, rhombic plates and 8 9 cubes have also been reported. The melting-point is about 1520 C. of it lies The density varies with the method preparation ; usually 10 between 4-87 and 4*53, but decomposition of the pentachloride in air and a the of which varied subsequent ignitionu gave product density between 4*88 and 5*05, and hydrolysis of the oxychloride followed by 12 ignition gave samples the densities of which were as high as 5-26. The crystalline modifications have densities varying between 4-57 and 13 476. The specific heat of niobium pentoxide between and 100 C. 14 is 0*1184, and increases at higher temperatures. The magnetic sus- 15 16 ceptibility and electrical conductivity have been measured. When

1 Muthmann, Weiss, and Riedelbauch, Annakn, 1907, 355, 63, 2 Zoe. Wohler, Pogg. Annalen, 1839, 48, 92; Rose, ibid., 1861, 112, 549; Ott, cit. ; Weiss and Landeoker, Zeitsch. anorg. Chem., 1909, 64, 66. 3 Read, J. Chem. Soc., 1894, 65, 313. 4 5 BQhm, Zeitsch. anorg. Chem., 1925, 149, 217. Rose, kc. cit. 6 Chim. 34 Ebelmen, Compt. rend., 1851, 33, 525 ; Ann. Phys., 1851, [Hi], 33, ; Mallard, 7 612, Ann. Mines, 1887, [viii], 12, 427. Nordenskiold, Pogg. Annalen, 1861, 114, 8 Knop, Annalen, 1871, 159, 56; Zeitsch. Kryst. Min., 1887, 12, 610. 9 Ruff, Seiferheld, and Suda, Zeitsch. anorg. Chem., 1913, 82, 397. 10 cit. Marignac, Ann. Chim. Phys., 1866, [iv], 8, 19; Ott, Zoc. ' l2 " Balke and Smith, J. Amer. Chem. Soc., 1908, 30, 1643. Rose, loc. cit, 18 cit. 207. Rose^Zoc ; Holmquist, Bull. Oeol Inst. Ups., 1896-97, 3, 14 Kriiss and Nilson, Zeitsch. physical. Chem., 1887, x, 391. 15 Berkmann and Zooher, ibid., 1926, 318. " 124, Friederich, Zeitsch. Phyaik, 1925, 51, 813. 156 VANADIUM, NIOBIUM, AND TANTALUM. of 1 gram of metallic niobium burns to the pentoxide, 2350 calories heat are evolved. 1 The calculated heat of formation of the pentoxide is given by the equation calories. 2Kb +|(50 2)=Nb 2 5 +4! 41,330 and the Niobium pentoxide is reduced to the dioxide, NbO 2, heated in a stream of at sesquioxide, Nb 2 3, on being strongly hydrogen 2 the carbon electric vacuum furnace ordinary pressures. Reduction in mixture of the lower oxides.3 at high temperatures and pressures yields a into the Ammonia at a white heat brings about partial conversion nitride.4 Heated in a current of hydrogen chloride, the oxide is com- and a white pletely volatilised without undergoing reduction, yields 5 .a?HCL Vanadium V , hydrochloride, Nb 2O5 pentoxide, 2 5 molybdenum also volatil- W0 , are trioxide, Mo0 3, and tungsten trioxide, 3 completely ised when heated in hydrogen chloride. The oxide is readily attacked by carbon tetrachloride vapour at 220 C. with formation of niobium tantalum is unaffected under the same con- pentachloride ; pentoxide 6 ditions. Sulphur monochloride and phosphorus pentachloride produce the and the pentachloride or oxychloride, according to conditions, with chlorine reacts similarly if the pentoxide is previously mixed an charcoal. Hydrogen sulphide and carbon bisulphide vapour yield is towards oxysulphide at high temperatures. Niobium pentoxide stable and reduction light, but becomes markedly photo-sensitive undergoes in the presence of certain organic liquids and reducing solutions, to some extent on the particularly glycerol. The process depends zirconium presence of impurities, notably stannic and tungstic acids, 7 compounds, and titanic acid. insoluble After being strongly ignited, niobium pentoxide becomes in all acids other than hydrofluoric acid, but is dissolved by molten borax. potassium hydrogen sulphate, ammonium hydrogen sulphate, and It is also insoluble in solutions of alkalis, but is converted into the alkali niobates by fusion with alkali hydroxides and carbonates. Hydrates of Niobium Pentoxide. Colloidal Niobium Pentoxide. Niobium pentoxide does not combine directly with water to form acids of definite composition. Two hydrates of the oxide, namely, 8 their exist- 3Nb 2O5.4H 2O and 3Nb 2 6.7H2O, have been reported, but ence is very improbable. The term niobic acid is applied to the more or less hydrated pentoxide. When niobium pentachloride or niobium is with excess of water there is oxytrichloride, NbOCl 3, hydrolysed 9 produced a white, amorphous, hydrated gel, which can also be obtained alkali niobates 10 by the action of sulphuric acid or hydrochloric acid on ; the precipitate is redissolved by excess of acid. Similar solutions are

Muthmann, Weiss, and Riedelbaueh, Annalen, 1907, 355, 84. Compare Nichols, Phys. Review, 1925, pi], 25, 376; Newbery and Pring, Proc. Roy floe., 1916, [A], 92, 276. Ruff, Seiferheld, and Suda, Zeitsch. anorg. Chem., 1913, 82, 397. Rose, Pogg. Annalen, 1860, .-in, 426. Hall and Smith, /. Arner. Chem. Soc., 1905, 27, 1389 ; Proc. Amer. Phil 8oc., 1905, 44, 197. According to Smith and Haas (Zeitsch. anorg. CJiem., 1894, 7, 96), the white product is a hydroehloride of niobium dioxide, and has the formula 2Nb0 2.HCL3H 20. 6 Ruff and Thomas, Zeitech. anorg. Chem., 1926, 156, 213. 7 Renz, Helv. Chim. Acta, 1921, 4, 961. 8 Sautesson, Bull Soc. chim., 1875, pi], 24, 38, 67. 9 Marignac, Ann. Chim. Phys., 1866, [iv], 8, 46 ; Rose, Pogg. Annakn, 1861, 112, 557. 10 Rose, foe. cit. ; Weinland and Storz, ZeitscL anorg. Ghem., 1907, 54, 232. COMPOUNDS OF NIOBIUM. 157 obtained by fusing a niobate with potassium hydrogen sulphate and the melt with water 1 the extracting ; extract, however, readily clouds, with precipitation of the acid. The can be dried at room to a mass at gel temperature horny ; 100 it C, becomes a white powder which still contains varying proof to the of the it portions water, according history sample ; is completely dehydrated at about 300 C. The gel is insoluble in water, but on being washed undergoes peptisation and passes through the filter- paper. This can be prevented by the addition of a small quantity of an electrolyte (for example, ammonium nitrate or ammonium carbonate) to the water. The gel is only slightly soluble in concentrated hydrochloric acid, with probable formation of niobium oxytrichloride, but the residue, after decantation, undergoes peptisation very readily to yield a clear hydrosol from which all the hydrochloric acid can be 2 removed by dialysis. The hydrosol prepared in this manner clouds if allowed to stand overnight or if boiled. The gel is more readily soluble in acid 3 concentrated sulphuric ; the solution remains clear 4 on being diluted, but it is precipitated by sulphur dioxide. The gel dissolves also in hydrobromic acid and perchloric acid, and is readily taken up by hydrofluoric acid. Hydrosols of both niobium pentoxide and tantalum pentoxide have been prepared by fusing each of these oxides with alkali in a silver crucible, dissolving the melt in water and dialysing the product for about ten days. Concentration over sulphuric acid yields a hydrosol 2*571 of niobium litre the containing grams pentoxide per ; dispersed phase is negatively charged. The sols so obtained are quite stable, and are not coagulated even when heated, but they are precipitated by all strong electrolytes except bases, which impart stability through preferential adsorption of hydroxyl ions. They are very sensitive to 5 chloride, sulphate, sulphite and nitrate ions. Hydrosols of niobium pentoxide and tantalum pentoxide differ in their behaviour towards carbon dioxide ; whereas the tantalum pentoxide coagulates and is precipitated fairly rapidly, the niobium pentoxide is not precipitated after a day tinder the same conditions. This difference in respective rates of coagulation has been used to separate niobium from tantalum, but the separation is not quantitative (see p. 129). The remarkable optical properties displayed by hydrosols of vanadium pentoxide (see p. 58) have not hitherto been observed with niobium pentoxide or tantalum pentoxide. In addition to dissolving in acids, freshly precipitated niobium pentoxide dissolves in caustic soda and in caustic potash, and hence it appears to possess weakly amphoteric character. Its colloidal state in solution has hitherto prevented any direct determination of its basicity 6 or acidity. According to Weinland and Storz it is comparable to silicic acid, and is more strongly acid than titanic acid. That niobic acid is a very weak acid is shown by the readiness with which the niobates are (a) hydrolysed and (&) decomposed by mineral acids, 1 and- Rose, Pogg. Annalen, 1861, 112, 551 ; 1861, 113, 109; Weiss Landecker, anorg. Chem., 1909, 64, 66. 2 Wohler, Pogg. Annakn, 1839, 48, 92 ; Marignao, Ann. Ghim. Phys., 1866, pv]> 8, 8 Compare Kiehl and Hart, J. Amer. CTusm. Soc., 1928, 50, 1613. 4 Weiss and Landecker, Zoc. c& 5 Hauser and Lewite, Zeitsch. angew. Chew., 1912, 25, 100. 6 Weinland and Storz, foe. e#. 158 VAJSTADIUM, NIOBIUM, AND TANTALUM. carbon with sulphur dioxide, and even so weak an acid as dioxide, of on the other hand, precipitation of niobic acid. Solutions vanadates, acid vanadates. on being treated with dilute acids yield the complex of Niobic acid combines in varying proportions with basic oxides forms condensed com- metals, which suggests that niobium pentoxide It is most how- plex anions containing several niobium atoms. probable, chemical but ever, that many of these niobates are not distinct identities, insolu- consist of isomorphous mixtures of simple salts. Their general which bility in water and the readiness with they undergo hydrolysis in contact with water have prevented any close investigation into their constitution, and hence into the constitution of niobic acid. with Niobic acid possesses the ability to form complex poly-acids and oxalic There are other acids (for example, with tungstic acids). by no means as many of these known as in the case of vanadic acid, as they have not hitherto received much attention.

NIOBATES. The alkali niobates are most conveniently prepared by the action of caustic alkalis on niobic acid or on solutions of niobium oxytrifluoride. Other compounds of niobic acid and bases are generally prepared by fusing niobic acid with the oxide, hydroxide, carbonate, or other salt of the metal. Occasionally double decomposition of a soluble alkali niobate and a soluble salt of the metal has been employed. 1 Larsson's method consists in precipitating a solution of potassium is fused for niobate with a salt of a metal ; the dried precipitate thirty-six hours at a high temperature with boric acid, and the melt is boiled with water to which hydrochloric acid has been added. The residue consists of crystals of the insoluble niobate of the metal, usually the metaniobate. Niobates obtained by any of these methods contain the basic oxide and acid oxide in proportions which vary from 1:4 to 5:1. The molecular complexities of most of them, as well as the complexity of not understood in the ions present in their solutions, are at present ; many cases they are probably isomorphous mixtures of simpler salts. A revision of their composition is necessary. Many crystalline niobates are known, however, the composition of which appears to correspond

to that of metaniobates, R"NbO 3 or R* 2O.Nb 2 5, and hexaniobates,

. R' R*8Nb6 O19 or 4R' 2O.3Nb 2 5 A few pyroniobates, 4Nb 2 7 or also The R" 2R" 2O.Nb 2 5, have been prepared. orthoniobates, 3NbO 4 are or 8R"2O.Nb 2 5 , like the orthovanadates, very unstable and but little known, although their existence has been recognised in minerals.

Niobates are usually insoluble in water ; even many of the alkali niobates are insoluble. Insoluble alkali niobates are formed when niobium pentoxide is fused with only small proportions of alkali carbonate. 7 : 6 Sodium niobate, 7Na 2O.6Nb aO5.o2H 2O, is insoluble in solutions of sodium-ion and is therefore high concentration, precipitated , by the addition of sodium salts to solutions of potassium niobate.

Ammonium niobates are unknown ; when excess of an ammonium salt is added to a solution of an alkali niobate, a voluminous* precipitate is thrown down which is rapidly decomposed by water to yield niobic 1 Larsson, Zeitsch. anorg. Chem., 1896, 12, 197. COMPOUNDS OF NIOBIUM. 159

1 acid. This instability of ammonium salts is consistent with the weak- ness of niobic acid, which is also shown in the ease with which niobates are decomposed by water and acids to precipitate niobic acid. Ar- senious, arsenic, oxalic, tartaric, citric and malonic acids do not pro- because of duce precipitates with solutions of alkali niobates, probably the formation of soluble complex salts. Phosphoric, hydrocyanic and of alkali acetic acids yield precipitates with concentrated solutions niobates. of The following niobates are known, but the constitution of some them is not beyond doubt : is obtained as a white Aluminium Niobate, Al 2 3.3Nb 2 5 .12H 20, precipitate by the action of a solution of alum on sodium metaniobate, 2 Na 2O.Nb 2O 5.TH 2O. Barium Niobate. A salt having the composition 7Ba0.6Nb 2 5 . is a hot solution of the 18H 2O prepared by treating corresponding sodium salt with dilute barium chloride solution.3

CdO.Nb O , is Cadmium Metaniobate.The anhydrous salt,^ 2 s It obtained fusing potassium niobate with cadmium chloride. by 4 is 5-93. forms yellowish-brown, glistening crystals, the density of which is thrown down as a The hydrated salt CdO.Nb 2 5 .3jH 2 pale yellow to meta- precipitate on adding a solution of a cadmium salt sodium niobate.5 Ccesium Niobates. Fusion of niobium pentoxide and caesium carbonate and extraction of the melt with water yields monoclinic crystals which have the 4Cs 0.3Nb .14H and which are iso- composition 2 2p5 20, morphous with the rubidium salt. Addition of alcohol to the aqueous s solution furnishes crystals of 7Cs 2O.6Nb 2 5 .30H 2O. is niobium Calcium Metaniobate, CaO.Nb 2O5 , prepared by fusing fluoride 7 pentoxide with calcium fluoride in an excess of potassium ; boric acid and the melt or the pyro-salt 2CaO.Nb 2 5 is fused with 8 extracted with water containing hydrochloric acid. It forms bright 9 red, long, thin, apparently rhombic needles, which are doubly refracting. Density 4-12 to 4-48. is with Calcium Pyroniobate, 2CaO.Nb 2 5 , prepared by fusing calcium chloride either niobium pentoxide or the precipitate obtained solution the by the action of calcium chloride on potassium niobate ; 10 melt is extracted with water containing hydrochloric acid, when the 11 Den- salt remains in small, colourless, glistening, prismatic crystals. sity at 17 C. 4-484. Isomorphous mixtures of calcium niobates and sodium niobates 12 occurs in the mineral have also been prepared. A calcium niobate pyrochlore. is the Cobalt Metaniobate, CoO.Nb 2 5 , prepared by heating together two oxides at about 1100 C., or by fusing the precipitate thrown down 1 Schoeller Weiss and Landecker, loc. tit.; Bullnheimer, Ghem. Zeit., 1900, 24, 870; and Jahn, Analyst, 1927, 52, 504. 2 Balke and Smith, J. Amer. Chem. Soc., 1908, 30, 1637. 3 loc. cit. Bedford, ibid., 1905, 27, 1218 ; see also Larsson, 4 Larsson, loc. cit. s Balke and Smith, loc. cit. ft Balke and Smith, loc. cit. 7 Annales Joly, Compt. rend., 1875, 81, 268; Chem. News, 1875, 32, 114 ; Scientifigues de Vtfcole Normale tiuperieure, Paris, 1877, [ii], 6, 172. 5 * 306. Larsson, loc. cit. Holmquist, Zeitech. Kryst. Min., 1899, 31, 10 loc. loc. cit. loc. cit. Joly, cit. ; Larsson, Compare Holmquist, 12 Holmquist, loc. cit. 160 VANADIUM, OTOBIUM, AND TANTALUM. by the action of cobalt nitrate on potassium niobate solution with boric 5-56.1 acid. It is a dark blue, crystalline powder. Density consists Copper Metaniobate. The anhydrous meta-salt CuO.Nb 2 5 to the corre- of black, glistening crystals which are prepared similarly 2 to sponding cobalt salt. Density 5*60. Addition of copper sulphate O.Nb O .7H an aqueous solution of sodium metaniobate, Na 2 2 5 2O, yields CuO.Nb O O. A a green precipitate of the hydrated compound 2 5 .3jH 2 3 dihydrate has been obtained by drying at 100 C. is obtained more Iron Niobates. Ferrous metaniobate, FeO.Nb 2O 5> and excess or less impure by fusing niobium pentoxide, ferrous fluoride, red heat. It of potassium chloride in a platinum crucible to a bright 4 forms A. ferric niobate, 2Fe 2t) 3.3Nb 205.8H 2O, long, steel-grey prisms. 5 has been obtained by the action of ferric chloride on sodium metaniobate. Iron niobates enter into the composition of the natural niobites and tantalites. is a substance Lithium Niobate, 7Li 2O.6Nb 2 5 .26H 20, crystalline which results from the action of lithium nitrate on a concentrated .5H O. G solution of potassium niobate, K 20.3Nb 2O5 2 is Magnesium Metaniobate (AnJiydrous), MgO.Nb 2 5 , prepared by the addition fusing with boric acid the precipitate thrown down on of magnesium chloride to a solution of potassium niobate. It yields 7 short, prismatic, doubly refracting crystals. The heptakydrate, is in white flakes the addition of MgO.Nb 2O5 .7H 2O, obtained by magnesium chloride to a solution of sodium metaniobate and drying on the 8 9 water-bath. Drying at 100 C. yields a tetrahydrate. of If the precipitate thrown down by the addition magnesium chloride to a solution of potassium niobate is fused with excess of 10 magnesium chloride, or if niobium pentoxide is fused with excess of 11 magnesium chloride under definite conditions, the salt 4MgO.Nb 2 5 is obtained. This compound is remarkable for the high proportion of to the basic oxide present. It is alternatively described according method of preparation as consisting of (a) very small, white, hexagonal plates or prisms, of density 4-43; (b) of colourless or pale yellow, broad, transparent, hexagonal leaves belonging to the rhombic system, density 4-37. It is only slowly attacked by acids. This salt is some- and times accompanied by crystals of the orthoniobate, 3MgO.Nb 2 5 , O was in one preparation magnesium pyrovanadate, 2MgO.Nb 2 5 , pro- 12 duced. The latter consisted of small, steel grey, glistening prisms, which became white when brought to a red heat. is Manganese Metaniobate, MnO.Nb 2O 5 , prepared by fusing together niobium pentoxide, manganese fluoride and potassium chloride. It 13 forms red, transparent, rhombic prisms, density 4'94. This salt is probably a constituent of the natural niobites (see p. 117). By fusing together niobium pentoxide, ferrous fluoride and manganese fluoride, Joly succeeded in preparing a substance which very closely resembled the natural niobites from Greenland. Analysis showed this substance to be an isomorphous mixture of ferrous metaniobate and manganese

1 Larsson, toe. cit. ; Hedvall, Zeitsch. anorg. Chem., 1915, 93, 391. 2 s 4 Larsson, toe. cit. Balke and Smith, toe. cit. Joly, toe. cit. 5 Rose, Fogg. Annalen, 1861, 112, 480 ; 1861, 113, 292. 6 Balke and Smith, toe. cit. 7 8 * Larsson, toe. cit. Balke and Smith, toe. cit. Rose, toe. cit. 10 u l8 I8 Larsson, toe. cit. Joly, toe. G& Joly, toe. c& Joly, too. cit^ COMPOUNDS OF NIOBIUM. 161

of the is metaniobate, composition (|Fe0.p[nO.)Nb 2O5 , which approximately the composition of the niobites from Limoges. Another 3Mn0.5Nb O has been manganese niobate, 2 5 , obtained by adding manganese sulphate to potassium niobate and fusing the precipitate with boric acid. It consists of small, greyish-yellow, prismatic 1 rods, density 4-97. Niobate. A salt the Mercury having probable composition Hg 2O. Nb 2O5 .3H 2O has been obtained by the action of mercurous nitrate on sodium niobate solution.2 Potassium Niobates are generally produced by fusion of the metal or the pentoxide with potassium hydroxide, potassium carbonate, or 3 potassium nitrate, or by the action of solutions of caustic potash or of potassium carbonate on niobic acid.4 They are among the most stable of their solutions the niobates ; can be boiled without precipitation of the acid. On being treated with sodium salts, for example sodium chloride, sodium niobates are precipitated. a solution of is When potassium niobium oxyfluoride, K 2NbOF5 , boiled with potassium bicarbonate, a light, powdery, practically in-

: is soluble precipitate of the 1 3 salt, K 2O.3Nb 2O5 .5H 2O, thrown down.5

1 : 2 Potassium niobate, K 2O.2Nb 2O5.5jH 2O, remains undissolved in the crystalline residue left after fusing equimolecular proportions of niobic acid and potassium carbonate and extracting the melt with water.6

3 : Thin, transparent plates of the anhydrous 4 salt, 3K 2O.4Nb 2 6 , remain undissolved when niobic acid is fused with about twice its weight of potassium sulphate for several hours at a red heat and the melt is extracted with water. 7 is as Potassium metaniobate, KNbO 3 or K 2O.Nb 2O5 , obtained beautiful, straw-yellow, rectangular plates by fusing a mixture of carbonate and niobic acid equimolecular proportions of potassium ; in this case the niobic acid is previously fused with calcium fluoride. The crystals are extracted from the product with boiling, dilute sulphuric acid.8

8 : 7 Potassium niobate, 8K 2O.7Nb 2O5.32H 2O, precipitates out on slow evaporation of solutions of either the 4 : 3 salt or the 7 : 6 salt. It forms rhombic bipyramids which can be recrystallised unchanged ; of water are lost a : b : c=0-9584 : 1 : 0*7083. Twenty-three molecules at 100 C. It readily yields supersaturated solutions. When its aqueous solutions are treated with a current of carbon dioxide they 9 precipitate salts which contain a larger proportion of niobic acid. is 7 : 6 Potassium niobate, 7K 2O.6Nb 2 5.27H 2O, precipitated by the addition of alcohol to an aqueous solution of the 4 : 3 salt. The 10 product is redissolved in water and reprecipitated several times.

1 * Larason, loc. cit. Rose, loc. cit. 3 Soc. 431 ; Muth- Moissan, Compt. rend., 1901, 133, 20 ; Bull Mm., 1902, [iii], 27, Zeitsch. mann, Weiss, and Riedelbauch, Annalen, 1907, 355, 67 ; von Bolton, EUMrochem., 1907, 13, 147. * Zeitsch. 53. Rose, Pogg. AnncOen, 1861, 113, 109 ; Russ, aw>rg. Chem., 1902, 31, 8 20. Marfgnac, Ann. Ohim. Phys, t 1866, [iv], 8, 6 Santesson, Butt. Soc. Mm., 1875, [ii], 24, 53. 7 * Joly, loc. cit. Joly, loc. tit. 9 cit. J. Amer. Chem. Soc., 1908, 1637. Marignac, loc. ; Balke and Smith, 30, 10 Balke and Smith, loc. cit. VOL. vi. : in. H 162 VANADIUM, NIOBIUM, AND TANTALUM.

is the commonest 4 : 3 Potassium niobate, 4K O.8Nb 8 6.16H aO, 2 which of the potassium niobates. It forms large monoclinic pnsms - 2Os.loi2u are isomorphous with 4 : 3 potassium tantalate, 4K20.3Ta of Niobic acid is fused with from two to three times its weight potassium is no evolved ; carbonate in the blowpipe until carbon dioxide longer either the melt is extracted with water and the extract is evaporated effloresce in vacua or allowed to evaporate spontaneously. The crystals at 100 t. in the air and lose twelve molecules of water of crystallisation melt after At a red heat the salt becomes yellow but does not ; being soluble in water. 1 so heated it is only partially . is obtained in rhombic 3 : 2 Potassium niobate, 3K 20.2Nb 2 5.13H 20, to a solution of the crystals by adding excess of potassium hydroxide It effloresces in the air and 4 : 3 salt and evaporating slowly. rapidly C.2 loses seven molecules of water of crystallisation at 100 2K O.Nb 2O5 . 2 : 1 Potassium niobate or potassium pyroniobate, 2 the 1 : 2 salt HHgO, is an insoluble powder which is obtained by fusing the with with excess of potassium carbonate and extracting product water. 3

: niobate ortho-salt) 5 : 2 Potassium niobate and 3 1 potassium (the 4 have also been reported. with Rubidium Niobates. When niobium pentoxide is fused rubidium carbonate and the melt is extracted with water, fine, silky are left behind. Con- needles of composition 3Rb 20.4Nb 2O5.9jH aO 0.3Nb O . centration of the filtrate yields monoclinic crystals of 4Rb 2 2 5 with the tantalum salt 14H 2O, which are isomorphous corresponding a : b : c= and with the corresponding caesium niobate and tantalate. 53'. It loses some of its water on 0-8815 : 1 : 1*0491 ; /J=95 slowly 5 exposure to- air. is obtained as a white Silver Metaniobate, Ag aO.Nb 2 5 .2H 20, solution of precipitate by the addition of silver nitrate to an aqueous sodium metaniobate. It becomes pale yellow on being dried and 6 nitrate gradually darkens in sunlight. Addition of dilute silver the solution to a solution of 7Na 20.6Nb 2 5 precipitates corresponding 7 is a insoluble substance. silver salt, 7Ag 20,6Nb 2 5.5H 2O, which white, Sodium Niobates are comparable to the potassium niobates. They differ in that they can be precipitated from solutions of potassium niobates by the action of neutral sodium salts. The usual method of preparation consists in fusing niobium pentoxide with caustic soda in varying proportions and, after washing away excess of caustic soda, that crystallising the product from water. It is to be noted, however, this method, in the hands of different investigators, has yielded several sodium niobates which differ in the relative amounts of base and acid present. It appears that the only sodium niobates hitherto prepared which can be definitely regarded as chemical individuals are the meta-

: .3lH or salt, Na 2O.Nb 2O5.7H 2O, and the 7 6 salt, 7Na 20.6Nb 2 5 2 8 is : 3 salt 32H 20. The commonest potassium niobate the 4 ; 4:3 9 sodium niobate has not been prepared.

1 loc. kc. cit. Chem. 154. Marignac, cit. ; Balke and Smith, ; von John, News, 1909, 100, 3 Marignac, foe. cit. ; Hermann, J. prakt. Chem., 1871, 3, 419 ; compare Bedford, Amer. Chem. Soc., 1905, 27, 1218. 3 * 5 Santesson, loc. cit. Russ, loo. tit. Balke and Smith, loc. cit. 6 7 Balke and Smith, loc. cit. Bedford, loc. cit. 8 loc. cit. /. 1783. Bedford, ; Smith and Van Haagen, Amer. Chem. Soc., 1915, 37, 9 SchoeUer and Jahn, Analyst, 1926, 51, 613. COMPOUNDS OF NIOBIUM. 163

2 : 3 Sodium niobate, 2Na 2O.3Nb 2Os .9H 2O, is said to have been prepared in small crystals by fusing niobium pentoxide with caustic soda 1 2 and extracting the melt with boiling water, but Bedford was unable to obtain the compound by this process. Sodium Metaniobate, Na 2O.Nb 2O 5 or NaNbO 3 . Anhydrous sodium metaniobate is obtained in white, strongly refractive, cubic crystals by ignition of the 7 : 6 salt, or by fusing equimolecular proportions of niobic acid and sodium carbonate in a flux of sodium fluoride. Excess of sodium fluoride is removed from the melt with water, in which the 3 niobate crystals are insoluble. Density, 4-512 to 4-559. Hydrated sodium metaniobate, Na 2O.Nb 2O5 .6H 2O or 7H 2O, is contained in the residue obtained after a fused mixture of niobium pentoxide and caustic soda has been extracted with a small volume of cold water, or in the residue obtained after boiling niobium pentoxide with caustic soda solution. Extraction of these residues with hot water yields small, glistening, triclinic crystals, which have also been obtained by the slow, spontaneous evaporation of the mother-liquors from the 7 : 6 salt.

: b : : 1 : 0-8394 7'. a c=0-9559 ; a=71 20', =105 30', y=54 One gram of the hydrated salt dissolves in 75 cc. of water at 100 C. and in 200 cc. of water at 14 C. Aqueous solutions slowly become cloudy on standing in the air, probably through the action of carbon

1 : dioxide, which precipitates the 4 salt, Na 2O.4Nb 2O5.H 2(X Addition of alcohol to the aqueous solution yields the 7 : 6 salt,

7(Na 2O.Nb 2O 5 )=7Na 2O.6Nb 2O5 +Nb 2O5 - Niobic acid, or more probably acid niobates, remain in solution. On the other hand, by passing a stream of carbon dioxide through an aqueous solution of the 7 : 6 salt, the 1 : 1 salt or metaniobate is pre- these sodium niobates hence are convertible.4 cipitated ; two mutually 7 : 6 Sodium niobate, 7Na 2O.6Nb 2O6 .3lH 2O or 32H 2O or 36H 2O, is prepared (a) by adding a large excess of caustic soda to a solution the of potassium niobium oxyfluoride, K 2NbOF5 , washing heavy precipitate of sodium niobate with cold water and crystallising the product -f four from boiling water ; (b) by using niobium pentoxide with about times its weight of caustic soda or sodium carbonate, washing the melt with cold water to remove excess of caustic soda, and crystallising the water niobium with product from boiling ; (c) by fusing pentoxide potassium carbonate in a gold or platinum crucible and precipitating the aqueous extract with sodium chloride solution. A micro-crystalline powder is usually obtained, but the salt crystallises in prisms which can be dried in the air without undergoing decomposition. It is readily and completely soluble in water, the aqueous solutions appearing to contain the undecomposed salt, because on the addition of salts of barium, silver or zinc, the corresponding 7 : 6 niobates of these metals are precipitated. Treatment of the aqueous solution with carbon dioxide, however, precipitates the meta-salt, which is also obtained by spontaneous evaporation of the mother-liquors from which the 7 : 6 salt has been 5 crystallised. 2 1 Santesson, loc. cit. Bedford, loc. tit. 3 Holmquist, ZeitscTi. Kryst. Min., 1899, 31, 306. * cit. loc. cit. Balke and loc. cit. Smith Bose, loc. cit. ; Joly, loc. ; Santesson, ; Smith, ; and Van Haagen, loc. cit. 6 loc. cit. loc. cit. von loc. Hermann, loc. cit. ; Bedford, ; Balke and Smith, ; John, Zoo. loc. cit. cit. ; Smith and Van Haagen, cit. ; Schoeller and Jahn, 164 VANADIUM, NIOBIUM, AND TANTALUM.

2 3 4 1 : niobates have also been 3 : 4, 8 : 7, 6 : 5, 3 : 2, and 3 1 sodium are doubtful, reported, but their identities (X A double sodium ammonium niobate of probable formula 5(NH 4 ) 2 .30H or 2 5 -5H 2 has been pre- Na2O.24Nb 2O 5 2 [f (NH4 ) ?4Na 2]O.4Nb or ammonium chloride pared by the addition of ammonium sulphate 5 to a solution of sodium niobate. Similarly, addition of caustic potash which contains some caustic soda to a solution of potassium niobate has 0.3K O.3Nb .9H (X6 yielded a sodium potassium niobate, Na 2 2 2 5 2 is thorium Thorium Niobate, 5ThO 2.16Nb 2 5 , prepared by adding the with sulphate to a solution of potassium niobate, fusing precipitate acid. borax and boiling the melt with water containing hydrochloric needles 5-21. 7 It forms doubly refracting, prismatic ; density Yttrium Niobates. Lesson's method yields very small needles .3Nb 4-83. The of the composition Y 2 3 2 5 ; density metaniobate, is obtained a mixture of niobium Y 2O 3.Nb 2 5 , by fusing pentoxide chloride at a white heat and yttrium chloride in excess of potassium ; obtained the addition of chloride alternatively, the precipitate by yttrium to a solution of potassium niobate is fused with anhydrous yttrium chloride.8 It forms doubly refracting, white octahedra; density 5-52. This compound is of interest in that it is probably a constituent of the natural andfergusonites (see p. 120). yttrotantalites 9 is Larsson's method. Zinc Metaniobate, ZnO.Nb 2 5 , prepared by 5-69. Addition It forms brown, apparently rhombic aggregates ; density : 6 a of zinc sulphate to a solution of the 7 sodium salt yields white, .25H 0.10 insoluble precipitate of 7Zn0.6Nb 2 5 2 Zirconium Niobate. The salt Zr0 2.5Nb 2 5 has been obtained in to the thorium 5-14.11 cloudy, rod-like needles, similarly salt; density This salt is remarkable for its large content of acid oxide. In addition to the foregoing, more or less impure niobates of beryllium, 12 cerium, lanthanum and nickel have been prepared. Niobic acid also reacts with hydroxylamine. When the 4 : 3 with concentrated am- potassium niobate is digested for several days monium hydroxide and hydroxylamine hydrochloride, a precipitate is obtained which, after being washed and dried, has the composition

. same the 3NH 2OH.HNb0 3 The process frequently yields compound 0. These are both white 5NH 2OH.2HNbO 3.H 2 sparingly soluble, substances, which decompose explosively on being heated. In contact with water at ordinary temperatures some hydroxylamine and a little niobic acid pass into solution. Analogous compounds with hydroxylamine are also given by vanadic acid (see p. 75), phosphoric acid, 13 tungstic acid, etc. The double salts (see pp. 146-148 and 152) that are formed from nio- niobium with bium oxytrifluoride, NbOF 8, and oxytrichloride, NbOCl 3, the halides of the metals can in some cases also be looked upon as salts of niobium .2H niobium pentoxide. Sodium o%yfluoride9 2NaF.NbOF 3 2O, can be written Nb 2O5.4NaF.6HF.H 20. These double fluorides are hence alternatively styled fluoroacyniobates. This name is not to be

2 3 1 Hermann, loc. c& Rose, loc. cit. Bedford, loc. cit. 4 * 6 Joly, loc. cit. Rose, loc. cit. Marignac, loc. cit, 7 Larsson, Zeitech. anorg. Chem., 1896, 12, 197 ; Holmquist, loc. cit. 8 * loc. cit. loc. cit. loc. cit t Joly, -, Larsson, Larsson, 10 n Bedford, loc. cit. Larsson, loc. cit. ; Holmquist, loc. cit. 18 Larsson, loc. cit. 19 Hofmann and KoMschiitter, Zeitech. anorg. Chem., 1898, 16, 473. COMPOUNDS OF NIOBIUM. 165

preferred, because the ions which are formed in the solutions appear n/ to be of the type [NbOF 3.nF] . The above-mentioned sodium salt, for instance, most probably yields the complex ion [NbOF5]".

HETEROPOLY-NIOBATES. The oxalo-niobates constitute the only well-defined series of heteropoly-niobates. Indications of the formation of salts of other heteropoly- acids containing complex anions depend on the following observations : (a) Niobium pentoxide yields complex compounds with tungsten 1 trioxide, WOg. (b) Addition of potassium chromate to a solution of niobium oxy- 2 trichloride yields a yellow precipitate of complex constitution. (c) The precipitation of niobic acid is hindered in the presence of titanic acid.3 (d) Whereas niobic acid is readily precipitated from solutions of niobates by the action of very weak acids, arsenic acid, citric acid, tartaric acid and malonic acid behave like oxalic acid and do not 4 yield precipitates. (e) Addition of sodium phosphate to a solution of niobium oxytrichloride or phosphoric acid to a concentrated solution of an alkali niobate yields a precipitate which contains both phosphoric acid and niobic acid.5 (f) Many complex titano-niobates, silico-niobates, titano-silico~ niobates, uranyl-titanoniobates, silico-zircononiobates, and tantalo-niobates occur naturally. Oxalo-niobates or niobo-oxalates correspond to the vanado-oxalates, and contain both oxalic acid and niobic acid radicals in the complex anion. The only known series possesses the general formula 3R* 2O. Nb 2O5.6C 2O 3.#H 2O, where R stands for an alkali metal. The sodium, potassium and rubidium salts are prepared by fusing one molecular proportion of niobium pentoxide with three molecular proportions of the alkali carbonate in a platinum crucible. The aqueous extract of the melt Js poured into hot oxalic acid solution ; concentration and cooling, or addition of alcohol or acetone, then brings about precipitation of the complex salt. Comparison of the electrical conductivity measurements of solutions of the alkali oxalo-niobates with those of the alkali hydrogen oxalates determined under the same conditions indicates that the oxalo-niobates are hydrolysed in aqueous solution, and that their anions contain a complex oxalo-niobic acid radical.6 .6C .3H is Ammonium Oxalo-niobate, 3(NH4 ) 2O.Nb 2O5 2O 3 2O, prepared by fusing niobium pentoxide with potassium carbonate, dissolving the melt in water and precipitating the solution with hydrochloric acid. The hydrated niobic acid so obtained is then dissolved in a solution of ammonium hydrogen oxalate. Cooling, or addition of alcohol, yields beautiful, glistening crystals which, however, readily undergo hydrolysis in water with precipitation of niobic acid. Potassium Orocalo-niobate, 3K 2O.Nb 2Os.6C 2O 3.4H 2O, can be recrystal-

Smith, Proc. Amer. Phil. Soc., 1905, 44, 158 ; Gibbs, Amer. J. Set., 1877, [iii], 14, 63. Blomstrand, Acta Univ. Lund, 1864. Hall and Smith, Proc. Amer. Phil. Soc., 1905, 44, 195.

Hall, loc. cit. ; Weiss and Landecker, Zeitsch. anorg. Chem., 1909, 64, 100. Rose, Pogg. Annalen, 1861, 112, 480 ; 1861, 113, 292. Buss, Zeitech. anorg. Chem., 1902, 31, 42. 166 VANADIUM, NIOBIUM, AND TANTALUM. of acetone lised unchanged from water, and also separates on the addition water of to its aqueous solution. It loses two molecules of crystal- at lisation between 60 and 65 C. and the remainder at 150 ., heated in a stream of which temperature decomposition begins. When it leaves a residue chlorine, carbon tetrachloride, or hydrogen chloride, reacts acid of potassium chloride and niobium pentoxide. Its solution with salts of a towards phenolphthalein, and yields precipitates large salts. number of metals, but not with zinc, manganese or mercuric oxalates have been Attempts to prepare other potassium niobium

unsuccessful, . is obtained m Rubidium Oxalo-niolate, 3Rb 2O.Nb 2O 5.6C 2O 3.4H 2O, solutions with alcohol. small needles by precipitation from its aqueous It forms supersaturated solutions very readily. is similar to the Sodium Oxalo-niolate, 3Na 2O.Nb 2O5.6C 2O 3.8HA to form potassium salt, but displays a greater tendency supersaturated solutions. does not Barium Oxalo-niobate, 5BaO.Nb 2O 5.10C 2O 3.20H 2O, belong to the alkali series described above. It is prepared either by the addition of barium chloride to a solution of potassium oxalo-niobate, or by niobium digestion of a mixture of barium oxalate and hydrated pentoxide in a solution of oxalic acid. The crystals are soluble in oxalic acid solution but insoluble in water and in cold hydrochloric and nitric acids. Oxalo-niobic Acid. Attempts to prepare the normal oxalo-niobic the action of niobic acid on oxalic acid solutions, acid, Nb 2 (C 2O 4 )5 , by were not successful. When a large excess of oxalic acid was employed, were crystals of a compound having the composition Nb(C 2O 4H) 5 only niobic acid was the once obtained ; when an excess of employed, .C or O was formed. Both of these compound Nb 2 5 2O3.3H 2O 4H 2 1 compounds are unstable in the dry state, but in faintly acid solution oxalo-niobic acid is much more stable than oxalo-tantalic acid, which reaction is preferentially hydrolysed by the addition of tannin. This of has recently been recommended for the separation and estimation niobium and tantalum.2

PERNIOBIC ACID AND THE PERNIOBATES. Perniobic Acid. When niobic acid is treated with hydrogen peroxide it becomes yellow owing to the formation of a perniobic acid, isolated HNbO 4.#H 20. This compound has been by carefully treating one of the potassium perniobates, K4Nb 2On .3H 20, with sulphuric acid and dialysing the mixture until it was free from both potassium then sulphate and excess of sulphuric acid ; the gelatinous product was 3 dried over concentrated sulphuric acid. Perniobic acid, thus prepared, is an amorphous, yellow powder, insoluble in water. At 100 C. it is decomposed with liberation of oxygen. Dilute sulphuric acid is without action on it at ordinary temperatures, but on warming the mixture decomposition ensues with formation of hydrogen peroxide. Con- centrated sulphuric acid liberates ozonised oxygen. The acid is much more stable than pervanadic acid, which is consistent with the general rule that in the same group the stability of the per-acid increases with 1 Buss, foe. tit. ; Weiss and Lazidecker, Zeitech* anorg. Chem., 1909, 64, 76. 2 Powell and Schoeller, Analyst, 1925, 50, 485. 3 Melikoff and Pissarjewsky, Zeitsch. anorg. Chem., 1899, 20, 341. COMPOUNDS OF OTOBIIM. 167 increase in atomic weight of the element. The active oxygen content agrees with the formula HO O

so that perniobic acid can be looked upon as being derived from of hydrogen peroxide by replacement a hydrogen atom by the NbO' g group : .0 0. Nb(OH)5 +H2 2 =Nb0 2 2H+3H 2

A hydrosol of perniobic acid is obtained by adding 30 per cent. hydrogen peroxide to niobic acid and gently warming the mixture on the cooled is treated in the cold with more the water-bath ; product hydrogen peroxide, excess of which is removed by dialysis. The hydrosol has the consistency of glycerine. It is converted into a yellow 1 gel either by standing for a long time or by the action of electrolytes. Another compound of niobic acid and hydrogen peroxide, of doubtful constitution, has been prepared by treating a solution of niobic acid in concentrated hydrochloric acid with 3 per cent, hydrogen peroxide. The mixture is .allowed to stand overnight, and the yellow precipitate, after being washed with water and dried, has the composition Nb(OH)6 2 it loses water or Nb 2O 5 .H 2O 2.5H 2O. On being heated and oxygen. R* Perniobates. Perniobates having the general formula 3Nb08 , where R stands for an alkali metal, can be prepared by the action of excess of hydrogen peroxide on solutions of niobates which also contain addition of alcohol alkali hydroxide or alkali carbonate ; precipitates alcohol and the perniobates as white powders, which are washed with ether. The rubidium and caesium salts become dark on being exposed are to the air, but otherwise the perniobates are stable in air, and they not decomposed in cold aqueous solution. Addition of sulphuric acid does not yield a precipitate in the cold, but on boiling, a yellow precipi- down, and tate, which is probably niobium perhydroxide, is thrown 3 been oxygen is evolved. The following constitutional formula has ascribed to the perniobates : ROCK ROO

but the evidence for this formula is weak. In addition to this series O .3H 0. the only well-defined perniobate is the potassium salt, K4Nb 2 n 2 The following perniobates are known : is the Potassium Perniobate, K4Nb 9 11 .3H 2 9 prepared by treating and saturated aqueous extract from the fusion of niobium pentoxide excess of caustic potash with hydrogen peroxide in small quantity, more warming gently on the water-bath, and filtering; hydrogen with alcohol peroxide is added to the filtrate, which is precipitated ; solution with more the precipitate* is treated in aqueous hydrogen It forms a white peroxide, caustic potash, and alcohol. soluble, powder, dilute acid which evolves oxygen on being warmed ; sulphuric produces

1 457. Melikoff and Piasarjewsky, J. Rue*. Phy*. Chem. Boe>, 1903, 35 2 J. Amer. Chem. Soc., Hall and Smith, Proc. Amer. Phil Soc., 1905, 44, 209 ; 1905, 27, 1400. 3 Balke and Smith, ftttf., 1908, 30, 1657. 168 VANADIUM, NIOBIUM, AND TANTALUM. acid ozonised hydrogen peroxide, and concentrated sulphuric produces is : O : active oxygen. Analysis shows that the ratio K 2 Nb a 5 oxygen 2:1:4, from which result the salt can be looked upon as the potassium salt of pyro-perjiiobic acid, KOCX ,0 >Nb/|x :CK I o

KOC time it When the salt is allowed to remain in solution for some slowly the of which precipitates another potassium perniobate, composition formula KNb0 .#H 0. closely approximates to the 4 2 is the action of caustic Potassium Perniobate, K 3Nb08 , prepared by on 4 : 3 niobate. potash and 3 per cent, hydrogen peroxide potassium It yields tetragonal crystals. is obtained as a white the Sodium Perniobate, Na 3Nb08 , powder by on 7 : 6 sodium action of caustic soda and 3 per cent, hydrogen peroxide niobate. is obtained as a Rubidium Perniobate, Rb 3NbO8 , white, crystalline and excess of powder by the action of rubidium carbonate hydrogen niobate. peroxide on 4 : 3 rubidium is to the rubidium salt. C&siumPerniobate, Cs 8NbO 8 , prepared similarly : KCaNbO . The following double perniobates have also been isolated 8 .7H O 8 2O ; 4H 20; NaCaNb08 .4H 2 ; KMgNbO8 2 ; RbMgNbO .7jH .8H 0. NaMgNb08 .8H 2O ; CsMgNb08 2 Niobium Peroxyfiuorides or Fluoroxyperniobates. Like mobic have of acid, the alkali niobium oxyfluorides the^ property taking up active oxygen by reaction with hydrogen peroxide. Potassium Niobium Peroxyfluoride. When potassium niobium oxy- is dissolved in 8 cent, fluoride, K 2NbOF5 .H 2O, per hydrogen peroxide, niobium or fine, white, greasy leaflets of potassium peroxyfluoride .H O or 2KF.NbO F .H potassium fluoroxyperniobate, K 2Nb0 2Fs 3 2 3 20, are obtained. This compound is stable in dry air. At 100 C. it loses water and at 150 C. it loses oxygen. Its aqueous solutions decompose the is decreased the addition of only very slowly ; solubility by hydrogen 1 peroxide and increased by the addition of hydrofluoric acid. Sodium Niobium Peroxyfluoride or sodium fluoroxyperniobate y .H is the action Na 3NbO 2F6 .H 2O or 3NaF.NbO 2F 3 2O, prepared by of hydrogen peroxide on sodium niobium oxyfluoride, SNaF.NbOF e, It forms yellow, almost transparent crystals, which become cloudy 2 on exposure to the air. Rubidium Niobium Pero&yfluoride or rubidium fluoroxypernidbate, is obtained as RbgNbOjjFg.HaO or 2RbF.Nb0 2F 3.H 20, similarly, thin, 3 yellow plates, from rubidium niobium oxyfluoride, 2RbF.NbOF 3 .

NIOBIUM AND SULPHUR.

Sulphides. Definite sulphides of niobium have not been prepared. Solutions of alkali niobates do not yield precipitates with hydrogen

1 cti. Piccini, Z&itsch. anorg. C'hem., 1892, 2, 22 ; Hall and Smith, loc. 2 3 Balke and Smith, loc. cit. Balke and Smith, toe. ctf. COMPOUNDS OF NIOBIUM. 169

1 sulphide or alkali sulphides, but some evidence of the formation of sulphides in the dry way has been obtained. When metallic niobium is heated with sulphur, direct combination takes place with the forma- is 2 tion of a black powder, which probably the disulphide, NbS a. This compound is perhaps also formed by burning the hydride in sulphur 3 vapour, or by the action at high temperatures of the mixed vapours 4 of carbon bisulphide and hydrogen sulphide on niobium pentoxide. The analytical data for these preparations are not, however, conclusive. Oxysulphides. The action of carbon bisulphide vapour or hydrogen sulphide on niobium pentoxide, sodium niobate or niobium oxytrichloride gives rise to a black powder which assumes a metallic appearance on being rubbed, and which conducts electricity well. This is probably an oxysulphide of niobium, Nba-O^S^, but its exact composition varies with the experimental conditions, and its individuality is a 5 matter of doubt. Oxysulphides of niobium of doubtful composition, mixed with potassium sulphide, are also produced by the action of carbon 6 bisulphide vapour on potassium oxalo-niobate. Sulphates. Electrolytic reduction of solutions of niobic acid in sulphuric acid furnishes solutions which contain the niobium either as

Nb O or Nb , to the but no 2 3 3 ? according experimental conditions, sulphates of niobium have been isolated from these solutions. Reddish- brown crystals of an ammonium niobium sulphate, which have the probable composition (NH 4 ) 2S0 4.Nb 2 (S0 4 ) 3.6H 20, have, however, been prepared by electrolytic reduction, as well as an acid ammonium niobium .Nb .6H O. latter is sulphate, (NH 4 ) 2SO 4 2(SO 4 ) 3.H2SO 4 2 The a brown powder which is somewhat stable in a dry atmosphere. It dissolves in water to give a brown solution which oxidises rapidly, of becoming successively blue, green, and finally colourless ; addition ammonium hydroxide or hydrochloric acid produces a blue coloration 7 immediately. Evaporation of a solution of niobium oxysulphide in sulphuric acid C. a of the and heating to 340 yields compound composition Nb 2 6 .SO 3 , written .SO i.e. as which can be alternatively Nb0 2 4.NbO 2, hyponiobic 8 sulphate. At 420 C. the product has the composition 2Nb 2O 5 .S0 3.

NIOBIUM AND NITROGEN.

The absorption of nitrogen by metallic niobium under different conditions of temperature and pressure has not been investigated. Two substances have, however, been prepared, to which the formulae NbN and Nb 3N5 have been ascribed, from their analytical data. Niobium Mononitride, NbN, is obtained by heating a mixture of niobium pentoxide and lampblack in an atmosphere of nitrogen at

1 Weiss and Landecker, Zeitsch. anorg. Chem., 1909, 64, 72. 2 von Bolton, Zeitsch. Etektrochem., 1907, 13, 148. 3 Kriiss and Nilson, Ber., 1887, 20, 1691. 4 Biltz and Voigt, Zeitsch. anorg. Chem., 1922, 120, 75; Biltz and Gonder, Ber., Zeitsch. 81. 1907, 40, 4963 ; compare Buss, anorg. Chem., 1902, 31, 5 Sci. Rose, Pogg. Annalen, 1860, ill, 193 ; Delafontaine, Arch. phys. not., 1866, J. 95 [n], 27, 173 ; Eammelsberg, prabt. Chem., 1869, 108, ; Hermann, ibid., 1871, 3,

393 ; Biltz and Kircher, Ber., 1910, 43, 1645. 6 Buss, loc. cit. 7 Stahler, Ber., 1914, 47, 842; Ott, Dissertation (Munich, 1911); Zeitsch. Elektrochem., 1912, 18, 349. 8 Gmelin-Kraut, Handbuch der anorganischen Chemie (Heidelberg), 1912, 6, 233. 170 VANADIUM, NIOBIUM, AND TANTALUM.

is a temperatures between 1200 and 1300 C. It bright grey powder abs. 8-4, The with a lustre, melting at 2300 ; density specific yellowish and electrical resistance is 2x10-* ohms per cc. at room temperature 4 in all acids 4-5 X 10~ ohms per cc. at the melting-point. It is insoluble of its content, and in aqua-regia. On being heated it loses much nitrogen heated either even when heated in an atmosphere of nitrogen. When Its in air or mixed with oxide it forms niobium pentoxide. copper 1 the method. crystal structure has been studied by Debye-Scherrer been obtained the action Triniobium Pentanitride, Nb 3N5 , has by 2 but of ammonia gas on an ether solution of niobium pentachloride, it is most Finely powdered conveniently prepared synthetically. ^ 1000 C. in an niobium is subjected to prolonged heating at about It is a black which is stable atmosphere of pure, dry nitrogen. powder the in the air. On being heated in air it glows and forms pentoxide, It but it is not oxidised by lead chromate even at high temperatures. or acids. is not attacked by boiling water, hydrochloric, nitric sulphuric action on but molten Boiling caustic potash solutions are also without it, of ammonia.3 caustic potash decomposes it with the evolution Other nitrides of niobium of doubtful composition have been obtained 4 5 by the action of ammonia on niobium pentoxide, niobium oxychloride and niobium pentachloride. Ferrocyanides. As might be anticipated from its very weakly acid nature, niobium pentoxide does not give rise to any cyanides. By on solu- the action of hydrochloric acid and potassium ferrocyanide The tions of niobic acid, three brown powders have been obtained. to them : following are some of the formulae that have been allotted but Nb ; K 6 ] 6 .10H 2O; Etfb[Fe(CN)6 ] 2 ; K 2 12[Fe(CN)6] 9 (NbO)5 ,[Fe(CN) 6 the evidence for these is weak.

NIOBIUM AND CAEBON.

Metallic niobium in the molten state absorbs graphite slowly to insoluble in all yield carbides of unknown composition. These are acids, brittle hard scratch including hydrofluoric acid, and are and very ; they 7 with carbon in quartz and glass. Reduction of niobium pentoxide 8 the electric furnace gives rise to similar products. A definite carbide having the composition NbC has recently been prepared by heating mixed with in at 1200 C. niobium sesquioxide, Nb 2 3, carbon, hydrogen Its hard- It is a greyish-violet powder which melts at about 3650 abs. ness after fusion lies between 9 and 10. Its specific electrical resistance at ordinary temperatures is 1-47 X 10-* ohms per cc. Its density is 7-56.* The crystal structure has been studied by the Debye-Scherrer 1 Mederich and Sittig, Zeitech. anorg. Chem., 1925, 143, 293 ; JTriederich, Zeitsch, 268. Physik, 1925, 31, 813 ; Becker and Ebert, ibid., 1925, 31, 2 Hall and Smith, Proc. Amer. Phil Soc., 1905, 44, 203; J. Amer. Chem. Sec., 1905, 27, 1395. 3 92 Muthmann, Weiss, and Biedelbauch, Annakn, 1907, 355, ; Moissan, Compt. rend., 1901, 133, 20. * Rose, Pogg. Annakn, 1860, in, 426, 429. 5 Joly, Annaks Scientifiques de YEcok Normak jSuperieure, Paris, 1877, [ii], 6, 154. 6 Wyrouboff, Ann. Chim. Phys., 1876, [v], 8, 444; Atterberg, Bull. Soc. chim., 1875, [ii], 24, 355 ; compare Weiss and Landecker, Zeitsch. anorg. Chem., 1909, 64, 100. 7 Moissan, loc. cit.; Butt. Soc. chim., 1902, [iii], 27, 431. 8 von Bolton, Zeitsch. Ekktrochem., 1907, 13, 145. 9 Becker and Jfriederieli and Sittig, Zeitsch. anorg. Chem., 1925, 144, 169 ; compare Ebert, Zeitsch. Physib, 1925, 31, 268. COMPOUNDS OF NIOBIUM. 171 method. Long, dark blue needles of a carbide having the same com- a mixture of position were also obtained by heating potassium niobate, carbonate and charcoal to a tem- K 20.3Nb 2O 55 potassium sugar high was perature in a graphite crucible. The product purified by washing 1 with sulphuric acid and water. The last-mentioned reaction at lower temperatures has also both furnished greyish-violet crystalline niobium compounds containing 2 carbon and nitrogen, and which appear to contain varying proportions of nitride and carbide of niobium. A substance which had the probable composition NbgOg.NgC was obtained as a black powder by the action of cyanogen on niobium 3 pentoxide at a red heat. Niobo-occalates are described on p. 165.

1 VlScole Normale 6, 142. Joly, Annales Scientifiques de Superieure, Paris, 1877, [ii], 2 loc. cit. Soc. 506. Joly, ; Bull chim., 1876, [ii], 25, s Rose, Pogg. Annalen, 1860, in, 427. CHAPTER VIII. TANTALUM AND ITS ALLOYS.

Symbol, Ta. Atomic Weight, 181-3 (0=16). Preparation of Metallic Tantalum. For the industrial preparation of tantalum a tantalite is used which contains at least 60 per cent, titanium. 1 The of tantalum pentoxide and the minimum quantity of is with either caustic or finely pulverised mineral fused potash potassium case the extract contains hydrogen sulphate. In the former aqueous which potassium tantalate and potassium niobate, undergo hydrolysis niobic acids when boiled with and precipitate mixed tantalic and acid in the latter case the mixed tantalic and sulphuric or nitric ; niobic acids are left in the insoluble residue after extraction with water. 2 The niobium is removed by dissolving the mixed acids in hydrofluoric sufficient fluoride is then added to the double acid ; potassium produce which are then fluorides, K 2TaF7 and K 2NbOF5 .H 20, separated by Considerable lias been repeated fractional crystallisation. secrecy maintained concerning the details of the large-scale processes employed tantalum to convert potassium tantalum fluoride, or the pentoxide obtained from it by hydrolysis, into metallic tantalum which possesses that reduction good mechanical properties. It is understood, however, is effected by one of the following methods : (1) The double fluoride is at a the heated with potassium or sodium high temperature ; product is washed with water and mineral acid, pressed into rods and fused in of an electrically heated vacuum furnace, whereby all traces impurities 3 are removed and a ductile material is obtained. (2) The molten double a fluoride is electrolysed between an anode of impure tantalum and cathode of pure tantalum, or of other conductive and inert material, in a vessel made of tantalum pentoxide, magnesia, or other refractory substance.4 The product is fused in a vacuum furnace as in the previous process. Tantalum can also be prepared in small quantities by passing an alternating electric current through rods of tantalum dioxide (see p. 196)

1 Analytical data of samples of ore utilised on the commercial scale are set out on p. 119. By-products produced during the working-up of the rare earths for cerium and thorium compounds for use in the manufacture of incandescent mantles, as well as by-products from certain tin and tungsten ores, are also available as sources of tantalum. 2 The various methods available for the removal of metals other than niobium are discussed on pp. 124 et seq. 3 Beraelius, Pogg. Annalen, 1825, 4, 10; von Bolton, Zeitsch. Elektrochem.,' 1905, II, J. 45 ; German Pat. 155548 (1905) ; Chem. Soc., Abs., 1905, 88, [ii], 96 ; Balke, Chem. Met. Eng., 1922, 26, 1272; J. 2nd. Eng. Ghem., 1923, 15, 560. 4 U.S. Pat. 947983 (1910) ; Eng. Pat. 24234 (1906). 172 TANTALUM AND ITS ALLOYS. 173 sealed into a glass bulb in which a good vacuum is maintained during 1 the heating. This is not, however, an industrial process. The malleability and ductility of tantalum are destroyed by the of bodies 0-1 cent, of for presence of even traces foreign ; per carbon, instance, renders the material brittle. Older laboratory reactions which rise to more or less tantalum deficient in mechanical gave pure u properties consisted in reducing tantalum pentoxide with mixed 2 electric 3 metal" (see p. 184) or with carbon in the furnace; the equilibrium conditions of the reduction of tantalum pentoxide by carbon at 4 high temperatures have been investigated by Slade and Higson. The 5 thermite process yields an alloy of tantalum and aluminium. Colloidal Tantalum. A sol of tantalum has been prepared by sparking tantalum electrodes immersed in isobutyl alcohol using an induction coil. The sol appears brownish-black by transmitted light and black 6 reflected it for one to two weeks. by light ; keeps only Physical Properties.7 Tantalum is a white metal with a greyish tinge and is very similar to platinum in colour and general appearance. 8 When it is heated to 1600 C. in vacuo it assumes a crystalline form. Examination of the powdered metal by X-ray analysis has shown that the arrangement of the atoms is on the plan of a body-centred cube of side 3-272 A, obtained by dividing the space of a crystal into equal closely packed cubes and placing an atom at each cube corner and is A. each cube centre ; the distance between the nearest atoms 2-833 is 16-6 a drawn into wire The specific gravity of the fused metal ; sample 10 the calculated from 005 mm. diameter had a density of 16-5 ; density X-ray data is 17-09.11 in which Pure tantalum possesses valuable mechanical properties, in it closely resembles molybdenum and tungsten. It can be worked the cold state to a remarkable extent without being previously annealed, although it is subject to strain-hardening, much like copper and silver. Photomicrographs of cold-worked and annealed tantalum are given in the reference cited. 12 The metal can be rolled into sheet 0-1 mm. thick or 13 even less, and can be drawn into filament wire 0-03 mm. diameter. The ultimate tensile strength of the hard-drawn wire (0-08 mm. diameter) inch this is 93 kilograms per square millimetre or 57 tons per square ,* of the wire 150 figure increases as the diameter diminishes, reaching which to 160 kilograms per square millimetre for wire 0-05 mm. thick, or is considerably higher than the value for hard-drawn copper, nickel, tensile of a carbon steel 089 cent. platinum ; the strength containing per of elasti- of carbon is about 52 tons per square inch. Young's modulus in city for tantalum wire (0-08 mm. diameter) kilograms per square

1 Joe. tit. von Bolton, -,.,_ i and a Weiss and Aichel, Annalen, 1904, 337, 370 ; Muthmann, Weiss, Riedelbauch,

Soc. chim. 434. ., 1902, I34> 211 ; BuU. 9 1902, [ii], 27, * Slade and Higson, J. Ohem. Soc,, 1919, 115, 211. c%L Goldschmidt and Vautin, J. Soc. Ohem. Ind., 1898, 17, 543 ; von Bolton, loo. see also Pat. 25864 ; Irench Pat. Svedberg, Ber., 1906, 39, 1705 ; Eng. (1906) Pat. 281305 371799 (1906) ; German (1913). 722 Chem. News, 7 von Bolton, Zeitsch. Elektrochem., 1905, xi, 45, 503, ; Semens, Chem. Met. 1922, 1271. 1909, ioo, 223 ; Balke, Eng., 27, 240. s A photomicrograph is given by Jeffries, Engineering, 1918, 100, 1445 1921, 571. 9 Hull, Proc. Amer. Inst. fflec. Engineers, 1919, 38, ; Phys. Review, 17, See also Hinrichsen and Sahlbom, Ber., 1906, 39, 2605. 12 i8 Pat. 247507 Hull, foe. cit. Jeffries, he. tit. JSng. (1925). 174 VANADIUM, NIOBIUM, AOT) TANTALUM.

millimetre is 19,000. The Brinell hardness number is 46, compared with l to 300 for steels ; 290 for tungsten, 147 for molybdenum, and from 100 the hardness is increased by the addition of small quantities of silicon, carbon, boron, aluminium, tin or titanium, and by traces of oxygen. When heated to redness and hammered, the metal becomes so hard through 2 the formation of an oxide film that a diamond will not scratch it, and at 3 the same time it retains its toughness. When heated in a poor vacuum, is its tantalum becomes very brittle and easily powdered ; ductility to a white and toughness are restored by heating in a good vacuum 4 determined 5 the heat. The under pressure has been ; compressibility 6 fractional change in volume caused by 1 megabar pressure average 6 at 20 C. is 0-54 XlO- . between applications of 100 and 500 megabars The linear coefficient of expansion per C. from to 400 C. is approxi- 6 7 as determined with a Fizeau dilatometer ; this mately 6-46 XlO- , and enables tantalum to be figure is less than that given by platinum 8 over the to 50 C. fused into glass. An earlier determination range 6 of gave a linear expansion of 7*9 x!0~ . The cubical coefficient expan- 9 6 is the calculated in- sion is 24 X 10~ ; the atomic volume 10-9, and ternal pressure of the atom, using the last two figures, is 455,000 10 megabars. 8000 C. The melting-point of tantalum lies between 2850 and ; 11 12 13 the most recent determinations are : 2798 C., 2850 C., 2910 C., 14 15 3000 C., 3030 C. The specific heat in calories per gram per at degree between 16 and 100 C. is 0-0365, and increases higher the coefficient of thermal is 0-130 calories temperatures ; conductivity 16 per cc. at 17 C. and 0429 calories at 100 C. Tantalum displays the decreases with weak paramagnetism ; magnetic susceptibility 17 the temperature. The electrical behaviour of tantalum has received considerable attention because of its application in the manufacture of electric lamps. The electrical resistance of 1 cc. of a sample which had been well annealed by heating for a period of between 100 and 200 18 hours in vacuo at 1900 C. was 14-6 microhms at 20 C. ; the corresponding figure for copper is 1-87. The electrical resistance increases with increase in temperature; the variation over the range from

1 Engle, Trans. Amer. Inst. Min, Met. Eng., 1925, 71, 691. 2 Grove-Palmer, Metallurgist (Supplement to The Engineer), 1927, 3, 185. 3 The Engineer, 1908, 106, 297. 4 Tiede and Birnbrauer, Zeitsch. anorg. Chem., 19H, 87, 148. 6 Richards and Bartlett, J. Amer. Chem. Soc. s 1915, 480. " 37, 8 One megabar equals 0-987 of an atmosphere." 7 Disch, Zeitsch. Physik, 1921, 5, 173; compare Worthing, Phys. Review, 1926, 28, 200. 8 von Bolton, loc* cit. 9 Richards, J. Amer. Chem. Soc., 1924, 46, 1419. 10 Richards, ibid., 1926, 48, 3076. 11 Forsythe, Astrophys. J., 1911, 34, 353; Engineering, 1912, 93, 189. 12 Pirani and Meyer, Verb. Dent, physikal Ges., 1911, 13, 540 ; Engineering, 1912, 94, 364. 13 Waidner and Burgess, J. Physique, 1907, 6, 380. 14 Pirani, Verh. Deut. physikal Ges., 1910, 12, 301. 5 * Worthing, Phys. Review, 1926, 28, 181. 16 Barratt loc. cit. and Winter, Ann. Physik, 1925, [iv], 77, 5 ; see also Worthing, 17 Loring, Chem. News, 1914, 109, 122; Owen, Ann. Physik, 1912, [iv], 37, 657, 666, 698. 18 Zeitsch. Pirani, Elektrochem., 1907, 13, 344 ; compare Pecheux, Compt. rend., 1911, 153, 1140 ; Schulze, Zeitsch. fur. Metallbunde, 1923, 15, 33. TANTALUM AND ITS ALLOYS. 175

180 to 2000 C. has been measured by Pirani.1 At 2000 C. the resistance is more than six times that at room temperatures ; at 180 C. it is reduced to approximately one-third. The influence on the electrical conductivity of pressures varying from 700 to 2000 atmospheres 2 has also been measured. For an investigation of the Hall effect and allied phenomena, see the reference cited.3 Tantalum has been employed as one of the elements in thermocouples, with copper or tungsten as the other element 4 a ; tantalum/tungsten thermocouple is stated to be four times as sensitive as platinum/platinum-rhodium. Optical Properties, The refractive index of tantalum is 2-05, the coefficient of absorption 2-31, and the reflexion capacity 43*8 per 5 cent, when measured with yellow light of wave-length A=5790. The spectral emissivity and the radiation intensity and their variation with temperature have been measured by Worthing. 6 For a comparison of the radiation constants of tantalum, platinum, osmium and carbon, see the references cited. 7 The flame spectrum of tantalum between 8 carbon electrodes consists of a blue cone with a reddish-yellow edge. The strongest lines in the arc spectrum of tantalum from wave-length 7000 A to wave-length 3000 A, and their relative intensities, are set out 9 in the table on p. 176. The spark spectrum of tantalum has been examined from wave-length 10 4700 A to wave length 2180 A. The strongest lines and their relative intensities are set out in the table on p. 176. The visible portion of the spark spectrum between tantalum electrodes is very weak, and the individual lines are measureable only with diffi- 11 culty. The lines between A=2000 and A=4700 in the spectrum of the spark discharge under water are the same as in air, unlike most other metals.12 In order to be able to establish spectrographically the presence of traces of a foreign element in a substance, de Gramont determined which of the lines in the spectrum produced by a condensed spark discharge are the last to disappear as the quantity of foreign element is gradually reduced. The ultimate lines given by tantalum in this manner 13 have the following 'wave-lengths, expressed in Angstrom units : (a) In

1 Pirani, Verh. Deut. physical. Ges., 1910, 12, 335; see also Pecheux, loc. cit.; Worthing, Phys. Review, 1926, 28, 197; Beckman, Physical. Zeitech., 1917, 18, 507; Proc. Holborn, Ann. Physik, 1919, 59, 145 ; Bridgman, Nat. Acad. Sci., 1917, 3, 10. 2 Beckmann, loc. cit. 3 Smith, Phys. Review, 1916, 8, 79. * Zeitsch. tech. 488. Worthing, ibid., 1912, 34, 153 ; Monigina, Phys., 1926, 7, s Wartenberg, Verh. Deut. physikal Ges., 1910, 12, 105. 6 Worthing, Phys. Review, 1926, 28, 174. 7 No. Coblentz, Bulletin Bureau of Standards, Washington, 1909, 5, 3, 339 ; Jolley, The Electrician, 1909, 63, 700, 755 ; Lavender, ibid., 1909, 64, 306 ; Paterson, ibid., 1916, 77, 822. 8 Mott, Trans. Amer. EUctrochem. Soc., 1917, 31, 272. 9 Eder and Valenta, Sitzungsber. K. AJcad. Wiss. Wien, 1910, 119, 582; Exner and 85 Riitten Haschek, ibid., 1898, 107, 813 ; Josewski, Zeitsch. wiss. Photochem., 1918, 17, ; in the Elements and Morsch, ibid., 1905, 3, 181 ; see also Stanley, Line* Arc Spectra of (Adam Hilger, Ltd., London, 1911). 10 Exner and Haschek, loc. cit. ; B.A* Reports, 1908, 119. 11 Eder and Valenta, Sitzungsber. K. Akad. Wiss. Wien, 1909, 118, 1902. 12 Zeitech. wiss. Konen and Finger, Zeitsch. ElMrochem., 1909, 15, 165 ; Konen, Trans. Soc. Photochem., 1909, 7, 329 ; compare Allin and Ireton, Roy. Canada, 1927, iii], 21, 127. 38 de Gramont, Compt.rend., 1920, 171, 1106; Twyman, Wa^kngth Tablesfor Spectrum Analysis (Adam Hilger, Ltd., London), 1923, 84. 176 VANADIUM, NIOBIUM, AND TANTALUM.

ARC SPECTRUM OF TANTALUM.

SPARK SPECTRUM OF TANTALUM. TANTALUM AND ITS ALLOYS. 177 the visible 6045-5 and 5997-4 region, ; (b) using a crown Uviol spectrograph, 3631-9, 3406-9, 3318-8, 3311-2 ; (c) a using quartz speetroeranh* L ' 2963-3. The of tantalum 1 X-ray spectra have been investigated. The emission of electrons from tantalum when heated to high temperatures 2 has received considerable investigation. Electron emission from the 3 cold metal has been studied by Rother, and the arrangement of electron 4 groups in the atom by Lessheim and Samuel. Tantalum is not radioactive.5 6 Chemical Properties. Tantalum is not affected by air or moisture at ordinary temperatures and does not "rust." When the metal is heated in the form of sheet or thick wire in air it becomes yellow at about 400 7 and with C., increasing temperature blue, and finally black. Above a dull red heat a film of the white pentoxide is produced, which to a large extent prevents further oxidation. Very thin tantalum wire can, however, be ignited in air by a match. In oxygen tantalum wire glows without flame at a white heat, and yields the pentoxide if the pressure of oxygen is greater than 20 mm. The reaction,

appears to be reversible, and proceeds completely from right to left this in vacuo at high temperatures ; enables pure tantalum metal to be from the 172 produced directly pentoxide (see p. ). Tantalum absorbs large volumes of hydrogen when heated in the gas, and yields a brittle product even when the amount of hydrogen present is less than 0-1 cent. 8 the absorbed is per ; gas completely removed by fusion in a good vacuum. Tantalum also absorbs nitrogen and, in minute quanti- 9 ties, helium and argon. It reacts slowly with sulphur and selenium, with formation of the and selenide probable sulphide ; hydrogen 10 sulphide is without action at 600 C. Tantalum is attacked readily 11 by fluorine, and burns when gently heated in chlorine, the penta-

1 For the results of the examination of K, L, and M series of the high-frequency spectra of tantalum, see the following references. K series : Cabrera, Compt. rend., 1107 and 1923, 176, 740 ; Rechou, ibid., 1925, 180, ; Stephenson Cork, Phys. Review, : Phil. 1926, 27, 138, 530. L series Moseley, Mag., 1914, [vi], 27, 710 ; Siegbahn and Friman, ibid., 1916, [Vi], 32, 47 ; Ann. Physik, 1916, [iv], 49, 616 ; Schror, ibid., 1926, 77 and [iv], 80, 297 ; Coster, Compt. rend., 1921, 173, ; Auger Dauvillier, ibid., 1923,

176, 1297 ; Coster, Phys. Review, 1922, pi], 19, 20 ; Cork, ibid., 1923, [ii], 21, 326 ; Wennerldf, Zeitsch. Physik, 1927, 41, 524; Nishina, Phil. Mag., 1925, [vi], 49, 521. M series : Stenstrom, Ann. Physik, 1918, [iv], 57, 347 ; Dauvillier, Compt. rend., 1926, Zeitsch. 65. 183, 193 ; Hjalmar, Physik, 1923, 15, 2 Harker and Kaye, Proc. Eoy. Soc., 1913, A, 88, 522 ; Horton and Davies, ibid., J. Amer. Ghem. 1919, A, 95, 333 ; Roy, ibid., 1926, A, 112, 599 ; Rodebush, /Soc., 1923, and 338 45, 997 ; Dushman, Rowe, Ewald, KLdner, Phys. Review, 1925, 25, ; Dushman, Zeitsch. 342 Ann. ibid., 1923, 21, 623 ; Suhrmann, Physik, 1923, 13, 17, ; Hiittemann, ibid. 609 Electrical Physik, 1914, [iv], 52, 816 ; Spanner, 9 1924, 75, ; Hyde, World, 1910, 55, 1654. Rother, Ann. Physik, 1926, 81, 317. Lessheim and Samuel, Zeitsch. Physik, 1926, 40, 220. Harkins and Proc. Acad. Strong, Amer. Chem. J., 1909, 42, 147 ; Guy, Washington, 1925, n, 628. von Bolton, Zeitsch. Elektrochem., 1905, II, 45, 722. Compare Balke, Chem. Met. Eng., 1922, 27, 1272. Osterheld, Zeitsch. Elektrochem., 1913, 19, 585. Sieverts and Bergner, Ber., 1912, 45, 2576. 10 Moissan, Compt. rend., 1902, 134, 212. " Ruff and Schiller, Ber., 1909, 42, 494.

VOL. VI. : III. 13 178 VAM1DIUM, NIOBIUM, AM) TANTALUM.

in case it can be made to combine with halide being formed each ; are also bromine, but does not react with iodine. Solutions of chlorine metal. The red-hot without action, but carbonyl chloride attacks the powder decomposes water with liberation of hydrogen. and m Tantalum is remarkably resistant to corrosion by acids, is, attacked fact, referred to as a "noble" metal. It is not by hydro- or dilute or chloric acid, nitric acid or aqua-regia, whether hot cold, is attacked hot dilute acid, but concentrated ; it not by sulphuric it It dissolves boiling concentrated sulphuric acid dissolves slowly. both metal and acid are in hydrofluoric acid, however, although when A of very pure, solution takes place only very slowly. mixture^ hydrofluoric acid and nitric acid attacks the metal rapidly, and in contact acid with platinum or carbon it is readily dissolved by hydrofluoric with evolution of hydrogen. Tantalum excellently withstands exposure effluents.1 to sea air, sea-water, sulphur dioxide, and mine even at 2000 C. 2 The vapours of the alkali metals are without action fusion with Boiling solutions of the alkalis attack tantalum slowly ; caustic potash in air yields a tantalate. Electromotive Behaviour. The behaviour of tantalum in cells is remarkable, and accounts for the rapid extension electrolytic " If a rod of tantalum of its application in electrolytic cell rectifiers." the is made the cathode in an electrolytic cell and a strip of platinum anode, and connection is made to the usual battery, the current passes If the con- through the cell as in the case of the commoner metals. nections to the battery are now reversed and the tantalum rod is made the anode, there is an instantaneous flow of current, but within a few seconds the current drops to a negligibly small value or ceases altogether. the current With an applied direct current E.M.F. of 75 volts, passing is less than 1 milliampre when sulphuric acid of the concentration 3 used in batteries is the Tantalum ordinarily storage "electrolyte. valve action" in that it therefore displays the phenomenon known as not in the allows the passage of an electric current in one direction but other. The effect is not restricted to sulphuric acid, but takes place 4 action in most electrolytes, excepting, however, fluorides. The valve is most probably due to the formation of an extremely thin layer of oxygen gas on the surface of the tantalum. This gas film penetrates 5 the blue, iridescent oxide layer produced by anodic oxidation of the tantalum when connection is made, and provides excellent electrical insulation for the whole anode.* If the applied E.M.F. is sufficiently increased, however, fine sparks become visible, and with further increase of the applied E.M.F. the insulation breaks down and an ap- current The voltage at which this occurs is termed preciable" passes. " the maximum voltage of the cell, and with an electrolyte which consists of a 0-02 per cent, solution of potassium carbonate it is 900 7 volts, which is a much higher figure than is given by other metals

1 lac. Grove-Palmer, cit. ; Guerfcler and Liepus, Zeitschrifi fur Metallkunde, 1925, 17, 310. 2 Fredenhagen, Physikal. Zeitech*, 1913, 14, 1047. 8 Balke, J. Ind. ting. Chem., 1923, 15, 561. * Compare Kuessner, Zeitscli. El&Hrockem., 1910, 16, 754. 5 See Dunham, Science, 1927, 65, 525. 6 Schulze, Ann. Phy&ik, 1907, 22, 543; 1909, 28, 787; Trans. Faraday Soc.> 1913, 266 ZeitscTi. 9, ,- Elefarochem., 1914, 20, 592 ; Zeitsch. Ptysik, 1921, 6, 237. 7 Schulze, Ann. Physik, 1907, 23, 226. TANTALUM AND ITS ALLOYS. 179

valve action, aluminium, niobium, showing namely, magnesium," bismuth, zirconium, zinc and cadmium.1 The maximum antimony," voltage of a metal depends on its physical condition, on the thickness of the oxide and gas films, and on the composition and ionic concentra- 2 it increases with tion of the electrolyte ; usually increasing dilution of the electrolyte. Valve action occurs with fused salts in much the as in solutions the same way aqueous ; maximum voltage is, however, 3 usually lower than the lowest maximum given by aqueous solutions. According to Schulze, valve action differs from the related phenomenon of passivity in that in the former case the oxide skin, 4 although very thin, is of definite thickness and prevents the passage of the current, whereas in the latter case the oxide film is an electrical conductor of molecular thickness. 5 If an electrolytic cell containing electrodes made of tantalum and lead is connected to a source of alternating current, the current passes freely during that portion of the cycle when the tantalum is the cathode, but little or no current passes when the tantalum is functioning as the anode. The result is that the current is converted into a alternating " pulsating direct current, and the cell is referred to as a rectifier." The electrolyte used industrially in rectifiers is accumulator acid, with the 6 addition of 1 per cent, ferrous sulphate. The current obtained can be used for charging storage batteries, for the electro-deposition of metals, and for other electrochemical operations which require direct 7 current. It has been found possible by using two tantalum electrodes in a single cell to rectify the current so that both half-periods pass in the same direction, giving rise to a non-pulsating and almost constant direct current. Tantalum has the great advantage over aluminium (which is the only other metal used to any extent as a rectifier) that it is much more resistant to the action of acids and alkalis, and hence enjoys a longer life and offers a much larger choice of electrolytes. Electrode Potential. This can be determined only in solutions of fluorides, since tantalum shows valve action in all other electrolytes, and even in the case of fluorides there is some reason to believe that at the oxide formation takes place, vitiating the results. The potential electrode

Tantalum (passive )/tantalum pentafluoride (0-006 mole.),

8 tantalum is obtained anodic is +1*537 volt ; H=zero. Passive by polarisation; the corresponding figure for the active material is +0*165 of the metal after volt. The potential obviously depends on the state ; which being rubbed with emery, tantalum gives a low energy potential

1 787 Schulze, Ann. Physik, 1907, 24, 43 ; 1908, 25, 775 ; 1908, 26, 372 ; 1909, 28, ;

1911, 34, 657 ; Zeitsch. Ekktrochem., 1908, 14, 333. 2 and Phil Schulze, Ann. Physik, 1$13, 41, 593 ; 1921, 65, 223 ; Taylor Inglis, Mag., 1 de and 1903, 5, 301 ; Bairsto and Mercer, Trans. Faraday Soc., 1911, 7, ; Bruyne Sanderson, ibid., 1927, 23, 42; U.8. Pats. 1602951 (1919), 1330581 (1920). 9 Schulze, Zeitsch. JSkktrochem., 1911, 17, 509. 4 Proa. 103. Schulze, ibid., 1912, 18, 22 ; compare JSTewbery, Moy. Soc., 1914, [A], 114, 6 Schulze, Trans. Faraday Soc., 1913, 9, 266. 6 The Engle, Trans. Amer. Inst. Min. Met. Eng., 1925, 71, 691 ; Anon., Engineer, 1925, 140, 235. 7 Balke, toe. tit. ; compare Schulze, Ann. Physik, 1914, 44, 1106. 8 Phil Hevesy and Slade, Zeitsch. Ekktrochem., 1912, 18, 1001 ; see also Hevesy, . Mag., 1912, [vi], 23, 643. 180 VANADIUM, NIOBIUM, AND TANTALUM.

1 slowly returns to the normal figure. The cathodic overvoltage in 2 normal sulphuric acid is O39 to 0*50 volt. Atomic Weight of Tantalum. The earliest investigations into 3 4 the atomic weight of tantalum were carried out by Berzelius, Rose, Hermann,5 and Blomstrand, 6 but the various values they obtained are now only of historical interest, as the materials used were not pure. 7 In 1866, Marignac made four analyses of pure potassium tantalum the salt with concentrated acid fluoride, K 2TaF7 , by heating sulphuric thus formed was to remove hydrofluoric acid ; the potassium sulphate extracted with water and the residue ignited and weighed as tantalum 8 pentoxide, with the following mean results : 2K TaF 100 2 7 = ;he ^ -Ta7T ^ K TaF 100 2 7 _ . ' " Tala "44-295

" Ta-183-7.Ta_ 183 . 7

were Four analyses of the corresponding ammonium salt, (NH4 ) 2TaF7 , also made, with the result :

JLa 2u5 DO-S

Despite the facts (a) that the figures for individual determinations units that the results as a differed by several whole and (ft) whole were obviously discordant, Marignac's work formed the basis for the accepted atomic weight of tantalum (namely, 183) for forty years. The suit- ability of the double fluorides for use in the determination of the atomic 9 weight has been questioned. 10 In 1906, Hinrichsen and Sahlbom used a very simple method. Metallic tantalum was converted directly into the oxide by heating in oxygen. The mean of five experiments gave the following ratio : 2Ta 81-902

The extreme values of the individual determinations still differed by more than a unit, and it is doubtful whether metallic tantalum can be obtained sufficiently pure for atomic weight determination. Balke in 1910 u tantalum to the hydrolysed pentachloride, TaCl^, a small nitric pentoxide, Ta2 s , with water and quantity of acid. The mean of eight experiments gave :

1 Schmidt, Zeitsch. physical Chem,, 1923, 106, 105. 2 Newbery, J. Chem. oc. t 1916, 109, 1108; compare Thiel and Hammerschmidt, Zeitsch. cworg. Chem., 1924, 132, 20 ; Osterheld, Zeitsch. fikktrochem., 1913, 19, 585. 8 * Berzelius, Pogg. Annakn, 1825, 4, 6. Kose, ibid., 1856/ 99, 78. 5 6 Hermann, J. yrakt. Ghem., 1857, 70, 193. Blomstrand, Acta Univ. Lund, 1864. 7 Chim. Marignac, Ann. Phys., 1866, [iv], 9, 251 ; Arch. Sci. phys. nat, 1866, [ii], 26, 89. 8 The fundamental values set out on p. viii of the General Introduction have been used in the calculations. The same fundamental values have been employed in the recalculation of the subsequent values for the atomic weight of tantalum mentioned in this section. 9 Proc. Smith, Amer. Phil Soc., 1905, 44, 151 ; Chem. News, 1905, 92, 210. 10 Hinrichsen and Sahlbom, JBer., 1906, 39, 2600. Balke, J. Amer. Chem. Soc., 1910, 32, 1127. TANTALUM AND ITS ALLOYS. 181 2TaCL 100 hence Ta=181-49. Ta 2O5 61-7355 The difference between the extreme values was 0-14. In the following year hydrolysis of the pentabromide with water and nitric acid was 1 used by Chapin and Smith, who from eight experiments obtained the ratio : 2*6212 hence Ta= 181-80. Ta 2 5 A The difference between the extreme values was 0-23. It will be observed that the value given by the pentabromide is appreciably higher than that given by the pentachloride, although the figures for individual determinations by each method were reasonably concordant. Sears 2 and Balke subsequently found that tantalum pentoxide is slightly volatile at ignition temperatures, and since it occludes nitric and other 3 acids it is useless for work requiring great accuracy. In a fresh attempt to establish the atomic weight, Sears and Balke treated the pentachloride with silver in the presence of hydrofluoric acid. Five experiments gave the mean ratio : TaCl 66-4338 5 hence ; Ta=181-05. 5Ag" 100 But the extreme values showed a difference of 0-46, from which it was concluded that the tantalum pentachloride used varied slightly in its composition, and that this salt is also unsuitable for use in the deter- 4 mination of the atomic weight. This conclusion has been confirmed. The values for the atomic weight of tantalum as determined by the various investigators since 1866 are set out in the following table: ATOMIC WEIGHT OF TANTALUM.

The International Committee on Atomic Weights adopted the value 181-5 in 1912, and this figure was altered to 181-3 in 1929. The decimal place is, however, obviously uncertain. 1 Chapin and Smith, J. Amer. Chem. Soc., 1911, 33, 1497. 2 Sears and Balke, ibid., 1915, 37, 833. 3 This observation affects the accuracy of all the foregoing investigations. 4 Sears, J. Amer. Chem. Soc., 1917, 39, 1582. 182 VANADIUM, OTOBIUM, A1STD TANTALUM.

Uses. Tantalum is used in the arts only in the metallic form, no for its the uses of the applications having been found compounds ; metal are restricted by its costliness, which arises from the necessity vacuum electric furnaces. In for preparing it in specially constructed than 1924 the world's annual production was probably not greater of 1 but new ten tons of ore, containing about 56 per cent, tantalum, The factor in the con- applications are being developed. largest an electrode in alter- sumption of tantalum is its recent application as in radio 2 It current electrolytic rectifiers, now familiar practice. nating 3 can also be used for the filaments of thermionic valves, for the electrodes 4 which its of in Rontgen tubes (for both of property absorbing gases and for the cathodes in electrochemical renders it particularly suitable), it is stated to have the over apparatus. For the last purpose advantage can be used in nitric acid platinum that it is mechanically stronger, or and solution, and does not alloy with zinc cadmium ; gold platinum cathode and removed can also be deposited on a tantalum subsequently 5 it has the serious disadvan- with aqua-regia. On the other hand, on of and tage that it tends to become brittle absorption hydrogen, results.6 Tantalum electrodes coated with then gives unsatisfactory 7 Other uses of tantalum platinum have been used with success. depend on its inertness under ordinary conditions and its resistance to attack by most acids. It is suitable for the manufacture of certain surgical and dental instruments, as it is not attacked by the ordinary antiseptics and chemicals used. A surface film which is almost as hard as agate can be produced on the metal by heat treatment. Analytical weights made with tantalum have received the approval of the International 8 Atomic Weight Committee. Its use is also suggested in place of the the manufacture of more expensive platinum for laboratory dishes, is com- crucibles, stirrers, etc., but this application restricted by the at which it to oxidise and its solu- paratively low temperature begins by these drawbacks it is recom- bility in hydrofluoric acid. To overcome mended that the metal be either alloyed, plated or sheathed with 9 be substituted. platinum, or that nickel-tantalum alloys Tantalum was used for several years for the filaments of incandescent efficient the electric lamps, as it was found to be electrically more for which had same candle-power than the carbon filaments previously life. In been employed, and it also enjoyed a longer 1912, however, it was in turn displaced by tungsten, which gave a still greater efficiency, and had the advantages of a higher melting-point in conjunction with 10 a higher ratio of hot to cold electrical resistance and lower cost. Tantalum electric lamps are no longer made, but it has been suggested

1 Taylor, Eng. and Min. J., 1924, 114, 842. 2 The 235. Robinson, Experimental Wireless, 1925, 2, 889 ; Anon., Engineer, 1925, 140, and Min. 817 Fleming, Engineering, 1909, 87, 883 ; Anon., Eng. J., 1923, 116, ; Balke, Industrial and Engineering Chemistry, 1923, 15, 561. 4 Siemens, Engineering, 1909, 87, 601. 5 565. Balke, loc. cit. ; Brunck, Chem. Zeit., 1912, 36, 1233 ; 1914, 38, Zeit. 989. Osterheld, Zeitsch. fflektrochem., 1913, 19, 585; Wegelin, Chem. 9 1913, 37, 7 Arndt, EleUrotech. ZeitscL, 1921, 42, 345. Nature, 1911, 87, 251. 9 Eng. Pats. 23050 (1912), 198246 (1922). 10 For data relating to the comparative behaviour of carbon, tantalum, tungsten, 822 and osmium lamps, see Balke, loc. cit. ; Paterson, The Electrician, 1916, 77, ; Crouch, 755 306 ibid., 1910, 64, 806 , JoUey, ibid., 1909, 63, 700, ; Lavender, ibid., 1909, 64, ; .211. Hirst, /. Inst. EUc. Eng., 1908, 41, 636 ; Swinburne, ibid., 1907, 38, TANTALUM AND ITS ALLOYS. 183

that they would be when it is preferable difficult to place a sufficient length of very fine wire to tungsten produce the resistance required on the circuit the ordinary ; higher electrical resistance of tantalum would enable a shorter wire or one of larger diameter to be employed, lantalum are also well lamps able to resist vibration and shock 3- It is that the use highly improbable of tantalum for electric lamps would have been followed a for by steady demand tantalum ores, because one of tantalum pound (avoirdupois) suffices for no less than 20 000 lamps Tantalum is stated to pentoxide be effective as a catalyst in the oxidation of hydrocarbons.2 Alloys. Tantalum yields alloys with a large number of other but their mechanical metals, properties and the systems produced have hitherto received little investigation. They are prepared by compress- the two metals and ing heating them to a high temperature in a good vacuum. or 5 Aluminium well alloys with up to about 3-5 per cent, of tantalum which has no on effect, however, the mechanical strength, ductility and of aluminium.* working properties Reduction of tantalum pentoxide the thermite by process yields hard, brittle alloys.* A substance the composition of which corresponds with the formula TaAL has been obtained by reducing tantalum potassium fluoride, K 2TaF7 , with aluminium at a filings high temperature. It is described as an iron- grey of crystalline powder, density 7-02, which is scarcely attacked by are Copper alloys mechanically strong and acid resisting/ Gold resemble alloys copper alloys. Gold-copper-tantalum alloys have also been made. 7 Iron with tantalum in all alloys proportions. Those alloys con- from 5 8 taining to 10 per cent, of iron are hard and ductile. Guillet 9 has examined the effect of tantalum up to 1-05 per cent, on the structure and mechanical properties of steels prepared with tantalum-iron in the electric furnace. In the case of normal steels the usual pearlitic structure was the present, effect of the tantalum being to produce more distribution of the regular pearlite. Quenched steels displayed their usual structure. tests Mechanical showed only slight increases in the breaking load, the limit of elasticity, and the resistance to shock. The same effects can be obtained readily by the addition of small proportions of nickel and other metals to the steel. The influence of tantalum on the characteristics in the 10 shearing test is also small. It appears, therefore, that the effect of tantalum on steel is too small to enable tantalum steels to attain any special importance. It has also been shown that no advantage is gained by substituting tantalum for nickel, cobalt or in 11 12 molybdenum high-speed steels. According to Guertler, tantalum 1 The Anon., Electrician, 1910, 65 ; Marine Supplement, 107. 2 UJS. Pats. 1636855 (1927), 1562480 (1925). 8 Schirmeister, Stahl und Eisen, 1915, 35, 999. Golcteehmidt J. and Vautin, Soc. Chem. 2nd., 1898, 17, 543. Arch. Set. Marignac, phys. nat., 1868, [ii], 31, 101. German Pat. 284241 (1913). Ibid. von Bolton, Zeitsch. EleJctrochem, 1905, 11. 45, 722. Guillet, GompL rend., 1907, 145, S27. 10 Portevin, Carnegie Scholarship Memoirs, Iron and Steel Institute, 1909, i, 277. 11 French and Digges, Trans. Amer. Soc. for Steel Treating, 1925, 8, 68L 12 /. Guertler, Giesserei, 1921, 8, 134 ; Iron Steel Inst, Ab&. t 1922, 105, 605. 1S4 VANADIUM, NlOBIUMj AND th in cast iron forms mixed crystals with the iron and precipitates carbon as graphite." " Iron undergoes cementation when heated in finely divided iron- cent, of tantalum the interior tantalum alloy containing about 30 per ; consists of a solid solution, which is bounded by a brilliant external layer the thickness of which increases with rise of temperature and with increase in the duration of heating. Copper and brass have been 1 similarly treated. Magnesium. A magnesium-tantalum alloy which contains about 3*5 per cent, of magnesium has been obtained by reducing tantalum 2 pentoxide with magnesium powder in a stream of hydrogen. Molybdenum alloys with tantalum in all proportions. Alloys containing from 10 to 40 per cent, tantalum have been suggested for the 3 construction of chemical and electrical apparatus. When the molybdenum content is less than 5 per cent., the product can be drawn into wire 0*1 mm. diameter. Nickel. Addition of from 5 to 10 per cent, of tantalum to nickel 4 considerably increases the resistance of the nickel to acids, and also its ductility. An alloy containing 30 per cent, of tantalum is not attacked by prolonged boiling with aqua-regia or other acids, and unlike tantalum it can be heated in air without undergoing oxidation ; it is also very tough and can be easily rolled, hammered, and drawn, 5 but may become brittle when strongly heated. Nickel-tantalum alloys which contain from 5 to 20 per cent, of chromium are also resistant 6 to heat and corrosion. An alloy which contains 75 per cent, of nickel, 11 per cent, of iron, and 14 per cent, of tantalum and niobium is claimed 7 to be suitable for electrical resistances and electrical heating apparatus. Platinum alloys containing from 0-5 to 20 per cent, of tantalum are hard, withstand heat, as well as the action of acids and fused potassium hydrogen sulphate, and are more resistant to the action of aqua- 8 regia than platinum. They possess the mechanical properties of are less platinum-iridium alloys and expensive ; the relative quantities, of tantalum and indium required to produce the same hardness andi mechanical resistance are stated to be 1 : 5. Platinum-tantalum alloys, hence have been recommended for various purposes in place of platinum or platinum-iridium. Tantalum can also be coated with platinum, andi 9 can then be utilised in high-temperature work. Silicon. Small amounts of silicon do not affect the ductility of 10 tantalum and increase its hardness. A substance the composition of which with the agrees formula TaSi 2 has been obtained by heating a mixture of tantalum pentoxide and silica in the presence of aluminium. It is described as a greyish-blue substance which forms four-sided prisms of density 8*8. It is stable in air, oxidises when heated in oxygen, and is insoluble in most acids it is 11 ; attacked by fused caustic soda. Laissus, Compt. rend., 1926, 182, 1152. Smith and Maas, Zeitsch. anorg. Ghem.* 1894> 7, %. U.8. Pat. 1385072 (1921). Bowe, Metal Industry, 1922, 20, 263.

Anon., Eng. and Min. J. t 1915, 99, 815. U.8. Pats. 1541301 (1925), 1588518 (1926). Canada Pat. 209342 (1921) ; UJ3. Pat. 1445253 (1923). French Pat. 477270 (1914) ; German Pat. 360006 (1919) ; Eng. Pat. 200074 (1923). Pat. 23050 Eng. (1912) ; U.S. Pat. 1180164 (1916). 10 von Bolton, toe. cit. 11 Honigscnmidt, Monatsh., 1907, 28, 1017. TAOTALUM AND ITS ALLOYS. 185

1 Tungsten alloys with tantalum in all proportions, Alloys of tungsten and tantalum which also contain cobalt, chromium or molyb- 2 denum have also been prepared. 3 Zirconium alloys can be heated to whiteness without undergoing oxidation.4 Sodium, potassium, mercury and silver do not alloy with tantalum even at 5 to high temperatures ; attempts prepare alloys with arsenic, antimony, lead, zinc and tellurium have also failed, but the formation of an alloy with silver, copper and tin for making a dental amalgam 6 with mercury has recently been claimed.

1 U.8. Pat. 1520794 (1925) ; von Bolton, loc. cit. 3 Eng. Pat. 152371 (1918) ; U.S. Pats. 1389679 and 1449338 (1923). 3 Canada Pat. 214118 (1921) ; U.8. Pats. 1248648 (1917), 1334089 (1920). 4 See also German Pat. 293952 (1913). 5 loc. cit. von Bolton, ; Hoissan, Compt. rend., 1902, 134, 411 ; Fredenhagen, Physikal. Zeitsch., 1913, 14, 1047. U.S. Pat. 1574714 (1926). CHAPTER IX. COMPOUNDS OF TANTALUM.

General. As in the case of niobium, the only well-defined tantalum compounds are those derived from the pentoxide, namely, the tantalates. Tantalum compounds display a much feebler tendency to undergo reduction than niobium compounds, and this is shown in the fact that and are and even the existence only two oxides, Ta 2 5 Ta0 2, known, 1 of the latter has recently been questioned. When niobium pentoxide is heated to redness in hydrogen at 1250 C. reduction to the sesquioxide, 2 similar Nb 2 3? ensues, but under conditions tantalum pentoxide 3 remains unchanged. Acid solutions of pentavalent niobium salts also undergo reduction with nascent hydrogen, whereas pentavalent tantalum salts are unaffected. It is of some interest to note, however, that evidence for the existence of a dichloride has recently been obtained. Tantalum pentoxide possesses only very weakly acidic properties. Its salts even with the strong alkalis are readily hydrolysed by boiling in aqueous solution. Its complex heteropoly-acids with other acids are ill-defined, but it takes up active oxygen to form a stable pertantalic acid, HTa04,#H 20. TANTALUM AND HYDROGEN.

Tantalum adsorbs hydrogen directly, or when it is used as the cathode 4 in the electrolysis of dilute sulphuric acid, but no definite hydrides have been isolated. One volume of tantalum in the form of wire, 0-3 mm. 5 diameter, takes up 775 volumes of hydrogen at room temperatures 6 and 46 volumes at about 800 C. The appended table gives the number of milligrams of hydrogen at 760 mm. pressure adsorbed by 100 grams 7 of tantalum at different temperatures. The amount of gas taken up decreases with increasing temperature, and at a given temperature (above 450 C.) is proportional to the square root of the gas pressure. The curve produced is similar to that given by palladium. Most of the hydrogen is expelled by heating to redness in

1 Eriederich and Sittig, Zeitsck. anorg. Chem., 1925, 145, 127. 2 Compare Buff, Seiferheld, and Suda, ibid., 1913, 82, 373. 3 A hydrated tantalum sesquioxide, Ta2 3.a;HaO, is stated to be formed by the addition of caustic soda to solid tantalum trioJhloride (Ruff and Thomas, Zeit&ch. anorg. Chem. 9

1925, 148, 3 ; Ber., 1922, 55, 1473). 4 von Zeitsch. Bolton, fflektrochem., 1905, II, 50 ; Osterheld, ibid., 1913, 19, 585 ;

and J. Atner. Chem. Soc. t 1529 Harding Smith, 1918, 40, ,* Coehn and Baumgarten, Zeitsch. physikal. Chem., 1927, 130, 545, 6 Sieverts and Bergner, .Ber., 1911, 44, 2394. 6 Thiel and Hammerschmidt, Zeitsch. anorg. Chem., 1923, 132, 15. 7 Sieverts and Bergner, loc. cit. 186 COMPOUNDS OF TANTALUM. 187

ADSORPTION OF HYDROGEN BY TANTALUM.

a vacuum, but the small remaining quantity is removed only by fusing the material in a vacuum in the electric furnace. All the metallic tantalum now made is subjected to the latter treatment so as to drive off occluded gases and other volatile impurities. When heated in hydrogen, tantalum wire undergoes a structural brittle and it retains alteration, becoming crystalline ; these properties after the hydrogen has been removed by heating to a high temperature 1 in vacuo. According to earlier investigators, the hydrogen absorbed at high temperatures is chemically combined with the tantalum, and it is stated that a hydride can also be obtained by the action of hydrogen on tantalum pentachloride.

Tantalum and the Halogens. The halides and oxyhalides of tantalum are set out in the following table : HALIDES AND OXYHALIDES OF TANTALUM.

* These compounds have not been isolated in the free state.

The pentavalent halides are the most stable, but even these can be prepared only in the dry way because of the readiness with which they the undergo hydrolysis. The trichloride is obtained by reduction of the same pentachloride with a powdered metal (lead, aluminium, zinc) ; 2 process has also given a dichloride and perhaps a tetrachloride, but their formation awaits independent confirmation. The preparation of the Moroatiti HTagCl^HgO, is of interest in that corresponding

1 and Riedel- Pirani, Zeitech. EUMrochem., 1905, n, 555 ; compare Muthmann, Weiss, bauch, AnnaUn, 1907, 355, 91. 2 1. Ruff and Thomas, er., 1922, 55, [B], 1466 ; Zeitsch. ammg. Chem., 1925, 148, 188 VANADIUM, NIOBIUM, AND TANTALUM.

Cl . chloroacids of molybdenum, HMogCl^HgO, and of tungsten, HWo 3 7 is some- 4H 20, have been obtained. The formation of the pentiodide as iodine what remarkable ; niobium has not yet yielded any compounds, and vanadium has given only the tri-iodide. It is unusual for a metal the falling in Groups IV. to VIII. to form an iodine derivative in which maximum valency of the group is displayed.

TANTALUM AND FLUORINE. Metallic tantalum and tantalum pentoxide are both dissolved by hydrofluoric acid, but evaporation of the solutions yields a residue which consists either of a tantalum oxyfluoride of variable composition or of the hydrated pentoxide. is the known fluoride of Tantalum Pentafluoride, TaF5 , only tantalum, and has been successfully isolated by methods that avoid hydrolysis : (1) Tantalum and fluorine are brought into reaction exactly 1 as in the preparation of niobium pentafluoride. (2) Tantalum penta- the chloride is treated in the cold with dry hydrofluoric acid ; hydrochloric acid liberated and excess of hydrofluoric acid are evaporated off, and the resulting tantalum pentafluoride is purified by redistilla- 2 tion in a platinum crucible between 300 and 400 C. (8) The double barium tantalum 3BaF .2TaF is heated in a fluoride, ? 5 , very strongly 3 platinum tube, one end of which is kept cold. Tantalum pentafluoride forms hygroscopic, colourless, doubly refracting, tetragonal prisms which melt at 96*8 C. and boil between 229-2 and 229-5 C. at 760 mm. pressure. Its density varies between

4-981 at 15 C. and 4-744 at 19-5 C. It is soluble in water ; solution is followed by hydrolysis which, however, does not proceed so readily as with tantalum pentachloride, niobium pentafluoride, and vanadium 4 pentafluoride. The aqueous solution evolves hydrogen fluoride on being evaporated, and leaves an insoluble tantalum oxyfluoride which is converted into tantalum pentoxide on being ignited. Caustic alkalis and alkali carbonates in concentrated solution attack the pentafluoride vigorously, and yield an alkali tantalum oxyfluoride of composition 4R'F. Ta2O5 .2TaF5 . Dilute alkalis yield tantalic acid, while fusion with potassium fluoride yields the double fluoride 2KF.TaF5 . Tantalum pentafluoride is also soluble in cold sulphur monochloride, sulphuryl chloride, stannic chloride, arsenious chloride, antimony pentachloride, alcohol, chloroform, glacial acetic acid, and acetic anhydride. Double Fluorides of Tantalum Pentafluoride. When solutions of tantalum pentoxide in hydrofluoric acid are treated with solutions of the fluorides of the alkali (and other) metals, double fluorides are obtained which the possess general formula nR*F.TaF5 , where n usually varies between 1 and 3 in the ; most important series n=2. These double fluorides are much more stable than tantalum pentafluoride. They were among the first tantalum compounds to receive examination, 5 and still form an important class of tantalum compounds. A study of their isomorphism with the corresponding compounds of niobium

1 Ruff and Schiller, Eer., 1909, 42, 494; Schiller, Dissertation (Danzig, 1911). 2 Ruff and Schiller, Zeitsch. anorg. Chem., 1911, 72, 331. 3 Hahn and Puetter, ibid., 1923, 127, 153. 4 Hevesy and Slade, Zeitsch. EUktrochem., 1912, 18, 1001. 6 Berzelius, Annalen, 1825 and and 6 481 : Pogg. 1826, 4 ; Rose,...*,*,ibid., 1856, oo, Marignac, Ann. Chim. Phys., 1866, PV], 9, 249. COMPOUNDS OF TANTALUM. 189 " " enabled the formula of tantalic acid (and hence of other tantalum compounds) to be correctly established (see p. 123); precipitation of still potassium tantalum fluoride, 2KF.TaF5 , constitutes the classical method for the separation of tantalum from niobium (see p. 128), and analyses of this salt provided the first reliable data for the atomic x weight of tantalum. Balke repeated and confirmed the previous investigations of Marignac, and showed that tantalum pentafiuoride forms several double salts with almost every one of the alkali fluorides, from which it appears that the preparation of any particular double state is matter the fluoride in the pure not an easy ; purest to be prepared the hitherto are the potassium salt, 2KF.TaF5, and sodium salt, 3NaF. TaF5 . The greater stability of these double fluorides as compared with tantalum pentafluoride has led to the assumption that their solutions contain complex anions into which the tantalum enters. The two salts just mentioned ionise, for instance, thus :

K 2TaF7 =2K'+[TaF7 ]", Na 3TaF8 =3Na'+[TaF8]'".

When viewed from the point of view of the Werner co-ordination theory, it is observed that in these complexes the common co-ordina- is tion number 7, thus [TaF7]K 2 and [TaF7]Cu.4H 2O ; the co-ordina- FF "1 tion number 8 also thus and Ta S and less occurs, [TaF8 ]Na 3 Q (NH4 ) 3, thus frequently the co-ordination number 6, [TaF6 ](NH4 ). The following double fluorides with tantalum pentafluoride are known: Acid Tantalum Fluoride or tantalum hydrogen fluoride, HF.TaF5 . in clusters 6H 2O or HTaF6 .6H 2O, has recently been prepared of feathery crystals which melt at about 15 C. by dissolving tantalum pentoxide in hydrofluoric acid and crystallising at about 10 C. It can be 2 rise to the series . looked upon as the acid which gives R*F.TaF5 Ammonium Tantalum Fluorides. The compound NH 4F.TaF6 or of fluoride on acid tantalum NH 4TaF6 results from the action ammonium fluoride.3 When a solution of tantalum pentoxide in hydrofluoric acid is treated with gaseous ammonia and evaporated, the compound is obtained in or 2NH4F.TaF5 or (NH 4 ) 2TaF7 thin, right-angled plates, in flattened needles belonging to the tetragonal system. It can be dried at 100 C. without undergoing decomposition, is readily soluble in water, from which it can be recrystallised unchanged, but the aqueous solution on being boiled throws down a white precipitate. Its pre- 4 which paration in the perfectly pure state is difficult. A compound F.TaF or TaF has also has the approximate composition 3NH 4 5 (NH 4 ) 3 8 been obtained. 5

Barium Tantalum Fluoride, 3BaF2 .2TaF6 or Ba 3Ta2F16, separates action of barium as a white, micro-crystalline precipitate from the chloride on a solution of tantalum pentoxide in slight excess of hydrofluoric acid. 6

1 Balke, J. Amer. Chem. Soc., 1905, 27, 1140. 2 Hahn and Puetter, loo. tit. 3 Hahn and Puetter, foe. tit. 4 272. Berzelins, foe. tit. ; Marignac, Ann. CUm. Phys., 1866, [iv], 9, 5 Balke, loc. tit. 6 Hahn and Puetter, loc. tit. 190 VAJSTABIUM, NIOBIUM, AND TANTALUM.

Ccesium Tantalum Fluorides. Substances having the compositions are a mixture CsF.TaF5 and 2CsF.TaF5 prepared by crystallising ^of cesium fluoride and tantalum fluoride from dilute hydrofluoric acid, or of caesium fluoride using excess of tantalum fluoride respectively. and the latter thin The former yields glistening, rhomboliedral crystals, 1 15CsF.TaF has needles. A stable, crystalline salt of the composition 5 2 also been reported* ^ ,TaF .4H or CuTaF .4H O, can Copper Tantalum Fluoride, CuF 2 5 2 7 2 the action of be obtained in blue, transparent, rhombic crystals by oxide on a solution of tantalum pentoxide in excess of hydro- copper 3 fluoric acid. It is very readily soluble in water. is Lithium Tantalum Fluoride, LiF.TaF5 .2H 2 or LiTaF6 .2H 20, a of tantalum obtained in colourless, prismatic crystals when solution acid is treated with lithium pentoxide in excess of hydrofluoric ofirbonate is Potassium Tantalum Fluoride, 2KF.TaF5 or K2TaF7 or [TaF7]K 2, the commonest of the double fluorides, and is in fact one of the commonest of tantalum compounds. It is prepared by adding potassium fluoride to a warm solution of tantalum pentoxide in hydrofluoric acid,

KHF , or by boiling tantalic acid with potassium hydrogen fluoride, 2 of tantalum and cooling the product. The preparation potassium fluoride by this method has already been referred to in describing the tantalum It separation and estimation of niobium and (see pp. 128-9). can also be obtained by the action of caustic potash or potassium 5 carbonate on a solution of tantalum pentoxide in hydrofluoric acid. rhombic Potassium tantalum fluoride crystallises in small, thin^ com- needles, which are isomorphous with the corresponding niobium

. is as 4*56 and pound KgNbF7 The density variously reported being 5-24. 6 The salt is stable in dry air at ordinary temperatures. On being and heated the pure substance decrepitates, melts to a clear liquid, 7 leaves a blue, infusible mass. It dissolves in water, but hydrolysis In water takes place, the extent of which depends on the conditions. containing very little hydrofluoric acid, 1 part of the salt dissolves in at 15 C. in the of rather more 200 parts of water ; presence hydro- 8 fluoric acid 1 part dissolves in 150 to 160 parts of water, at 15 C. 9 Ruff and Schiller have shown that the solubility increases (1) with rise in temperature, (2) with increase in the concentration of hydrofluoric acid, (3) with decrease in the concentration of potassium fluoride. The aqueous solution reacts acid to litmus, and on being boiled pre- 10 cipitates a white, insoluble, potassium tantalum oxyfluoride, Ta 2 5 . this reaction is used to detect the 4KF.2TaF5 or Ta 2 5.2(2KF.TaF5 ) ; presence of tantalum in a niobium compound, since a solution of

1 Balke, J. Amer. Chem. Soc., 1905, 27, 1151. 3 Pennington, ibid., 1896, 18, 38. Balke (kc. ciL) was unable to obtain this very complex salt. 3 Marignac, Ann. CUm. Ptys., 1866, [iv], 9, 247. 4 Balke, loc. cit. 5 Berzellus, loc. tit. ; Marignac, loc. cit. 6 Gmelin-Kraut, Handbuch der anorganischen Chemie (Heidelberg), 1913, 6, 313. 7 Rose, loc. cit. ; Balke, loc. cit. ; Halm and Puetter, 7*>c. cit. 8 Marignac, loc. cit. 9 Buff and Schiller, Zeitech. anorg. Chem., 1911, 72, 342. 10 Compare Noyes and Bray, Qualitative Analysis for the Hare Elements (Maomillan, London), 1927, 106. COMPOUNDS OF TANTALUM. 191 does not throw a potassium niobium fluoride, K 2NbF7? down precipitate 1 on being boiled. Addition of caustic soda precipitates tantalic acid. is Rubidium Tantalum Fluoride, 2RbF.TaF6 or Rb 2TaF7 , obtained in white needles similarly to the analogous potassium compound. It 2 dissolves in 40 parts of water. Sodium Tantalum Fluorides. Slow evaporation of a solution of sodium tantalate in hydrofluoric acid precipitates the two compounds, .H or TaF .H the 3NaF.TaF5 or Na 3TaF8 , and 2NaF.TaF5 2 Na 2 7 2O, 3 : b : former first. Na3TaF8 yields rhombic prisms, a c=0-6017-:l: 0-2799, and dissolves in from 20-5 to 20-9 parts of water at 25 C. rise to six-sided to the rhombic Na2TaF7 ,H2 gives plates belonging

: : it its of system, a : b : c=0*838 1 1-274; loses water crystallisation at 100 C., and can be heated to 150 C. without undergoing decomposition. Evaporation of its mother-liquors yields the salt NaF.TaF5 or NaTaF6 in regular, cubic crystals. Analogous sodium niobium fluorides have not been isolated. is in Thallium Tantalum Fluoride, 2TlF.TaF5 or Tl 2TaF7 , obtained glistening crystals by the action of thallium fluoride on tantalum 4 pentoxide in hydrofluoric acid. .TaF is the action of Zinc Tantalum Fluoride, ZnF 2 6 , prepared by zinc oxide on a solution of tantalum pentoxide in excess of hydrofluoric a mass.5 acid ; concentration yields hygroscopic, crystalline Tantalum pentafluoride also forms crystalline double salts with com- pyridine, methylamine, and other organic bases. The following 6 : .2H O pounds with pyridine have been prepared 3(C5H5N.HF).2TaF 5 2 ; C5H5N.HF.TaF5 . Oxyfluorides. Vanadium yields more or less stable oxyfluorides, and an oxyfluoride has been in the but of pentavalent niobium, NbOF 3 , prepared dry way, in the case of tantalum, free oxyfluorides are unknown. Tantalum does in combination with oxytrifluoride, TaOF 3, occur, however, ammonium and potassium fluorides in some double salts. The latter are not so numerous as the analogous niobium compounds, and differ in from them also in method of preparation, since they are not formed rise the double fluorides the presence of hydrofluoric acid, which gives to described above. On being boiled, the double oxyfluorides of tantalum or an undergo ready hydrolysis, and precipitate either tantaJic acid than in oxyfluoride in which the proportion of tantalic acid is greater the original salt. Ammonium Tantalum Oxyfluorides or ammonium fluoroxytantalates. "or A substance which has the composition 3NH4F.TaOF 3 6 has been obtained tantalic acid in a hot, con- [~TaQ l(NH4 ) 3 by dissolving centrated solution of ammonium fluoride and cooling, whereupon large 7 transparent octahedra are thrown down. This fluoride is isomorphous 1 Compare Powell and SchoeUer, Analyst, 1925, 50, 495. 2 lac. cit. B. F. Chem. News, 1905, 92, 209 ; Pennington, loo. cit. ; Balke, ; Smith, Proc. Amer. Phil Soc., 1904, 44, 151. * loc. foe. cit. Marigaae, cit, ; Balke, 4 Ephraim and Heymann, Ber., 1909, 42, 4461. 5 Marigrtac, loc. cit. * cit. Balke, loc. , , , . 7 de VEcok NormaU Supeneure, Joly, Gompt. rend., 1875, 81, 1266 ; Annales ficientifique* /, Amer. Chem. Soc., 1905, 1140. Paris, 1877, [ii], 6> 160 ; compare Balke, 27, 192 VANADIUM, NIOBIUM, AND TANTALUM. and with the both with the corresponding niobium salt, 3NH 4F,NbOF 3 , double fluorides of some metals in which the oxygen atom is substituted by another fluoride atom, for example ammonium titanium fluoride,

. the and 3NH 4F.TiF 4 Evaporation of ammonia from mother-liquors 1 cooling yields crystals of the compound 3NH 4F.HF.TaOF 3 . .Ta or Ta . Potassium Tantalum Oxyfluoride, 4KF.2TaF5 2 5 2 5

2KF.TaF , 2(2KF.TaF5 ).~-(See under Potassium Tantalum Fluoride, 6 have been on p. 190,) Two other potassium tantalum oxyfluorides 2 3 reported, 2KF.TaOF 3 and 3KF.TaOF 3 .

TANTALUM AND CHLORINE.

TaCl , Tantalum Dichloride, TaCl 2 , and Tantalum Trichloride, 3 are both stated to be obtained by reducing the pentachloride with chloride the is heated aluminium in the presence of aluminium ; product to 600 C. so as to volatilise the aluminium salt, and the residue is extracted with cold water. The trichloride dissolves readily and leaves 4 the dichloride as a dark olive-green powder, which oxidises on exposure to air and is converted into the pentoxide on being heated. The dichloride dissolves in warm water with evolution of hydrogen to form an oxychloride of trivalent tantalum. Cold dilute caustic soda solution dissolves it without evolution of hydrogen or alteration in valency, but on being warmed, the solution oxidises readily, evolves hydrogen, and an unstable brown lower oxide which is also thrown down Srecipitatesy the addition of ammonia to the aqueous solution of the dichloride, or by the action of nitric acid or other strong oxidising agent on the acid solution. The composition of the green powder and the existence of tantalum dichloride have been rendered doubtful by the investigations 5 of Lindner and Feit, who reduced tantalum pentachloride with lead powder at 600 C. in an atmosphere of nitrogen, and extracted the product with dilute hydrochloric acid, from which, after removal of lead with hydrogen sulphide, six-sided, dark green crystals of a chloro-acid having the composition HTa 3Cl7 .4H 2O were obtained. This compound loses only three of its water molecules at 205 C., and at slightly higher temperatures undergoes decomposition with evolution of hydrochloric acid. It contains a complex ion the chlorine atoms of which scarcely dissociate in alcohol solution in dissociation is ; water, followed by hydrolysis, with the splitting off of two molecules of hydrochloric acid. The co-ordinated formula is H[Ta3Cl7 .H 2O].3H 20, analogous to the formulae for the corresponding molybdenum and tungsten chloro- 6 7 acids. The formula was previously written both as TaCl 2.2H 2 and 8 as (Ta,Cl12 )Cl 2.7H 20. A study of the reactions of this chloro-acid of tantalum shows that in acid solution only one of the seven chlorine atoms is dissociated, and

1 Joly, loc. cit. 2 Hall and Smith, Proc. Amer. Phil 8oc., 1905, 44, 179. 3 Joly, loc. cit. 4 Ruff and Thomas, Zeitsch. anorg. Chem., 1925, 148, 1 ; Ber.9 1922, 55 B, 1466. 5 Lindner and Zeitsch. 66 Lindner Feit, anorg. Chem., 1924, 137, ,* and others, Ber., 1922, 55 B, 1458. 6 Lindner and loc. cit. Zeitsch, Feit, ; Lindner, anorg. Chem., 1927, 162, 203 ; compare. Ruff and Thomas, ibid., 1925, 148, 19. 7 Chabrie, Compt. rend., 1907, 144, 804. 8 Chapin, /. Amer. Chem. Soc., 1910, 32, 329. COMPOUNDS OF TANTALUM. 193 a number of compounds have been prepared which are derived from the chloro-acid by substitution either of this differently held chlorine atom l or of the co-ordinated water : Cl .Br.H is obtained as (i) A monobromochloro-acid, H[Ta 3 6 20].3H 20, green crystals by the action of hydrobromic acid on tantalum pentachloride. Cl .S0 is obtained (ii) A sulphatochloro-acid, H2[Ta 3 6 4].H 2 5 by treating tantalum pentachloride with dilute sulphuric acid. of the alcohol solution of the chloro-acid a (iii) Evaporation yields Cl brown resinous mass of composition H[Ta 3 6 .Cl.C 2H5QH].C 2H5OH. acid in or alcoholic (iv) With pyridine and hydrochloric aqueous are : N. solutions, deep green, crystalline compounds produced C5H5 C Cl .Br.C H All these coin- Cl .CLC H5 ; 6 3 6 5 5N]. H[Ta3 6 5 N] H5N.H[Ta _ pounds have the co-ordination number 8, and all of them contain the Cl as of the anion, which is a group (Ta 3 6 ) part complex probably Cl acid of derivative of the complex anion [Ta3 8]". The composi- is but a salt, tion H 2Ta 3Cla unknown, green, crystalline pyridiniwn Cl has been Other (C5H5N 2 )H 2[Ta 3 8 ).3H 2O, prepared. pyridinium 2 derivatives in which the co-ordination number is 7 or 9 are known. On being treated with aqueous caustic potash, the chloro-acid loses two of its chlorine atoms and yields a dark brown, amorphous precipi- in which the co-ordination tate of composition [Ta 3Cl5 (H 2O) 5].OH aq., number is 10. is the Tantalum Trichloride, TaCl 3 , prepared by reducing pentachloride with aluminium in the presence of aluminium chloride and is a dark heating the product to between 350 and 400 C.* It green without substance which yields intensely green aqueous solutions evolution of hydrogen. These solutions are fairly stable, and oxidise to air the rate of oxidation is accelerated only slowly on being exposed ; the addition of acids. by the presence of alkali and checked by sub- Tantalum Tetrachloride, TaCl 4. The formation of a green a tetrachloride stance the composition of which agreed with that of but from the was observed during the preparation of the trichloride, data available it was impossible to decide if it was a simple compound 4 or a mixture of the pentachloride and the trichloride. best of the chlorides Tantalum Pentachloride, TaCl55 is the known of tantalum. It has been by several methods : prepared 6 action of chlorine 5 or chloride on heated (i) By the carbonyl metallic tantalum. The pentachloride can be sublimed away. of carbon tetrachloride, 7 carbon tetrachloride and (ii) the action By 9 8 or chlorine, sulphur monochloride and chlorine, phosphorus pentachloride 10 on tantalum pentoxide.

2 i Compare Buff and Thomas, loc. tit. Lindner, loc. cit. * Buff and Thomas, loc. cit. Ruff and Thomas, er., 1922, 55 B, 1466. and Moissan, Com**- rend im, Berzelius, Pogg. Annalen, 1825 and 1826, 4 6; Lindner and Peit, Zeitsch. amorg. Chem., JSoc. 434 ,- 134, 211 ; Butt. Mm., 1902, [in], 27, 1924, 132, 10. 6 Lindner and others, loc. cit. 7 Hall and Proc. Amer. Ph%l.8oc., Demarcay, Oompt. rend., 1887, 104, 111 ; Smith, 277 ; Punk J. Chem, Soc., 1905, 1393 ; Chem. News, 1905, 92, 1905, 44, 202 ; Amor. 27, and Mederlander, Ber.f 1928, 61 B, 249, 1385. 330. Buff and Schffler, Zettsch. anorg. Chem., 1911, 72, Ann. Cfam. Phys., 1910, [vm], Hall, /. Amer. Chem. Soc., 1904, 26, 1243 ; Boimon, /. Amer. Ohem. So& 9 1915, 37, 835. 20, 566 ; Sears and Balke, 10 Pennington, ibid., 1896, 18, 64. 1 VOL. VI. : III. ^ 194 VANADIUM, NIOBIUM, AND TANTALUM. the action of chlorine on a heated mixture of tantalum (iii) By 1 pentoxide and sugar charcoal. obtained as a Tantalum pentachloride is usually yellow powder or sublimed. Its which forms a mass of white crystals on being melted 2 3 is as 211-3 C., 221 C., and between melting-point variously reported 6 4 24.1-6 5 and 233 C. Its 230 and 240 C., and its boiling-point as 7 at 360 C. is 12-8 density at 17 C. is 3-68, and its vapour density (air so that does not take place =1) ; TaCl5 requires 12-5, decomposition in the fused state is at this temperature. The electrical conductivity 6 for at 0-30X10- reciprocal ohms (the corresponding figure copper 4 is so that molten tantalum penta- ordinary temperatures 64X10 ), water. 8 chloride is an insulator of the order of the best conductivity sublimes in chlorine or in It is quite stable in dry air, and unchanged it emits a carbon dioxide. When brought into contact with water into tantalic acid and acid, hissing noise, and decomposes hydrochloric with evolution of heat. It is only sparingly soluble in hot concentrated water it dissolves hydrochloric acid, but with addition of completely throw down a even to an opalescent solution which does not precipitate attacks on being boiled. Concentrated hydrochloric acid it, hydrogen at the same chloride fumes being evolved, but the tantalic acid formed time is dissolved; the solution becomes cloudy on being boiled, and cooled.9 throws down a gelatinous precipitate of tantalic acid on being cold carbon Tantalum pentachloride is soluble in alcohol, chloroform, tetrachloride and carbon bisulphide; solutions in these solvents yield treated with crystalline addition compounds when organic compounds. have The following addition compounds with pyridine and piperidine 10 .6C N.2C OH. Rose 11 been isolated: TaCls.2C 5H5N ; TaCl5 5Hn 2H5 was unable to obtain double chlorides of tantalum pentachloride and the alkali chlorides which would correspond to the double fluorides. Cl Oxychlorides. A tantalum oxychloride of composition TaO 2 has been obtained by subliming tantalum pentachloride in a vacuum at 500 C.12 Various oxychlorides of unknown composition are stated to be obtained by the action of tantalum pentachloride on alcohol 13 solutions of hydrochloric acid. The following pyridine and quinoline 14 : C1.7C Ta O Cl . addition compounds have been prepared 4Ta0 2 5H5N ; 2 3 4 TaOCl N. 4(C5H6N.HC1) ; 2TaOCl 3.3(C5H6N.HCl).2C 2H5OH ; 3.2(C5H5 TaOCl N. HCl^CsHgOH; 2TaOCl 3.3(C5H5N.HCl).2C 2H6OH ; 3.2(C9H7 HC1).2C 2H5OH. TANTALUM AND BROMINE. Bromotantalum Bromide or tantalum dibromide. The anhydrous substance is unknown. A compound which has the com- or is position (Ta 3Br6 )Br.3jH 2 TaeBr14.TH2 obtained by reducing 1 Biltz and Voigt, Zeitech. anorg. Ckem., 1921, 120, 71 ; Rose and "Weber, Pogg. AnnaUn,

1846, 69, 115 ; 1853, 90, 458, 2 Deville and Troost, Compt. rend., 1865, 60, 1221 ; 1867, 64, 294. 3 * Hose, Pogg. Annalen, 1846, 69, 115. Biltz and Voigt, toe. cit. 5 8 Deville and Troost, loc. cit Lindner and Feit, loc. cit. 7 Balke, J. Amer. Chem. Soc., 1910, 32, 1131. 8 Biltz and Voigt, loc. cit. 9 575 Weinland and Zeitsch. Rose, Pogg. Annalen, 1856, 99, 66, ,- Storz, anorg. Chem., 1907, 54, 223. 10 Lindner and Felt, loc. cit. ; see also Funk and Niederlander, loc. cit. 11 12 Rose, loc. cit. Ruff and Thomas, Zeitsch. anorg. Ckem., 1925, 148, 1. 13 14 Weinland and Storz, loc. cit. Lindner and ITeit, loc. cit. COMPOUNDS OF TANTALUM. 195 tantalum pentabromide with sodium amalgam at a red heat in the of air the is absence ; product extracted with water which has been acidified with hydrobromic acid, and the extract concentrated. Minute, black, hexagonal crystals result, which give a dark green powder. They yield an intensely green aqueous solution which is apparently stable in air. Addition of ammonium hydroxide precipitates brown flakes of an unstable lower hydroxide of tantalum. When the compound containing seven atoms of bromine is treated with silver nitrate solution, only one atom of bromine is removed as silver bromide, which indicates that six atoms of bromine form part of a complex anion and do not undergo ionisation as Br' ions. Similarly, when*the aqueous solution of bromotantalum bromide is treated with equimolecular proportions of caustic soda in the cold, dark green, thin, hexagonal plates of bromotantalum hydroxide, (Ta 3Br6 )OH.5H 2O, are obtained. Evaporation of the hydroxide with hydrochloric acid yields bromotantalum chloride, (Ta 3Br6 )CL3jH 2O, which is very similar in appearance and general properties to bromotantalum bromide. Evaporation of the hydroxide with hydriodic acid gives rise to bromotantalum iodide, (Ta 3Br6 )I.3|H 2O, in long, hexagonal prisms. All these substances the Br contain group (Ta 3 6 ), which, however, has not been separately isolated. 1 It is important to note that recent investigation into the composition of the analogous chlorine compound of tantalum, viz. chlorotantalum chloride (see p. 192), has shown that the formula H.Ta 3Cl6 .C1.4H 2 is to the older formula Cl .C1.4H to be preferred Ta 3 6 20. By analogy, the of the bromine is composition compound H.Ta 3Bre .Br.4H 2O, and this assumption receives confirmation from the fact that the acid 2 H[Ta 3Br6 .Br.H 2O] has recently been prepared. The foregoing formulae for the hydroxy-, chloro- and iodo-derivatives also presumably require correction.

Tantalum Tribromide, TaBr 3 . A substance having this composition has not been definitely isolated and analysed, but it is stated to be formed during the reduction of tantalum pentabromide with hydrogen. It is a green powder which yields an intensely green aqueous solution in which the compound is only weakly ionised. The green is action of colour discharged by the oxidising agents ; addition of 3 ammonia precipitates characteristic brown flakes. TaBr can be distil- Tantalum Pentabromide, 5 , prepared either by 4 ling bromine on to powdered tantalum heated to between 260 and 300 C., or by passing bromine vapour over a heated mixture of freshly ignited tantalum pentoxide and sugar charcoal, all air being previously removed by means of carbon dioxide. The product is sublimed in an atmosphere of carbon dioxide to render it free from excess of bromine. The salt crystallises in long, yellow plates. The density of the sublimed material is 4*67. It melts at about 240 C. to a transparent, red liquid, and boils at about 320 C., giving a yellow vapour. When heated in hydrogen above its sublimation temperature, reduction to the metal and the lower bromides takes place. It fumes strongly in air, and is rapidly attacked by water, with precipitation of tantalic acid. Methyl and

1 Chapin, J. Amer. Chem. Soc., 1910, 32, 324* 2 Lindner, Zeitsch. anorg. Chem., 1927, 162, 209. 3 Van Haagen, Thesis, Univ. of Pennsylvania (1909). * Krishnaswami, Nature, 1928, 122, 845* 196 VANADIUM, NIOBIUM, AND TANTALUM. considerable evolution ethyl alcohols react with it readily, and with such 1 of heat that they take fire. Oxybromide. The formation of a yellow tantalum oxybromide, observed the distillation of tantalum TaOBr s, has been during penta- brornide. 2 No double salts with it are known.

TANTALUM AND IODINE. Tantalum and iodine do not combine directly even when heated tube.3 to together for eight hours at 280 C. in a sealed Attempts pre- a mixture of tantalum pare an iodide by the action of iodine on pentoxide and carbon were also unsuccessful. 4 been obtained Tantalum Pentiodide, TaI5 , has by distilling and tantalum pentabromide in a stream of dry hydrogen iodide purifying of carbon dioxide. It is the product by redistilling it in an atmosphere an almost black substance which melts to a brown liquid, and is decom- but not so posed by water similarly to the pentabromide, vigorously. Bromotantalum iodide has been referred to on p. 195. No oxyiodides of tantalum have been prepared. TANTALUM AND OXYGEN. been as a dark brown Tantalum Dioxide, Ta0 2, has prepared and powder by heating rods made of a mixture of tantalum pentoxide at about 1700 5 or the paraffin in powdered charcoal C., by reducing 6 It is stated to be pentoxide at a high temperature with magnesium. of the metals formed also during the electrolysis of solutions of salts 7 it takes using a tantalum anode. On being heated in air up oxygen in and forms the pentoxide. It is insoluble in acids, but dissolves molten potassium hydroxide to form potassium tantalate. is of the Tantalum Pentoxide, Ta 2 5 , one commonest compounds of tantalum. The anhydrous substance is produced by direct oxidation which is of the metal or by ignition of hydrated tantalum pentoxide, obtained by the methods described below. The removal of niobium and other metals has been described when dealing with the extraction of tantalum and niobium from their natural ores. Anhydrous tantalum pentoxide, as usually prepared, is a white, non- 8 white volatile, tasteless, odourless, amorphous powder, which remains 9 it and at high temperatures, When heated to dull redness glows assumes a crystalline form (rhombic prisms), isomorphous with niobium 10 takes when the sub- pentoxide ; the same change place amorphous 11 stance is fused with boric acid or microcosmic salt. Its melting-point

1 87 J. Ghem. 729 j Rose, Pogg. Anwlen, 1856, 99, ; Van Haagen, Amer. 8oc., 1910, 32, Chapin and Smith, ibid., 1911, 33 1499. 3 2 Chapin and Smith, foe. cit. Van Haagen, toe* cit. * cit. 211. Rose, foe. ; Moissan, Gompt. rend., 1902, 134, * and von Bolton, Zeitech. EUktrochem., 1905, u, 45 ; compare Friederich Sittig, Ghem. 397. foe. cit. ; Ruff, Seiferheld, and Suda, Zeitsch. anorg. 3 1913, 82, Smith and Maas, ibid., 1894, 7, 98. 7 Brunck, Ghem. Zeit., 1912, 36, 1533. 8 Compare Sears and Balke, /. Amer. Ghem. Soc., 1915, 37, 835. 9 Weiss and Landecker, Zeitsch. anorg. Ghem., 1909, 64, 81. * Tammann, Ann. Physik, 1902, [iv], 9, 249 ; Bohm, Zeitsch. anorg. Ghem., 1925, 149, 217.

i- 1 Ebelmen, Ann. Ghim. Phys., 1851, 33, 54 ; Mallard, Ann. Mines, 1887, [viii], 12,

427 ; Nordenskidld and Chydenius, Ofvers, K. Vet.-Akad. Fork., 1860, 3 ; Pogg. Annalen, Butt. Inst. 1860, no, 645 ; Holmquist, GeoL *7#*.,. 1896-97, 3, 205. COMPOUNDS OF TANTALUM. 197

has been determined under different conditions with discordant results : 1 2 1875 C., 1620 C. Its density is approximately 7-5, but the figure obtained varies \vith the method of preparation, increasing with rise 3 in "temperature of ignition and prolongation of duration of heating. 4 The electrical resistivity has been measured. One gram of metallic tantalum evolves 1373 calories of heat when burnt directly to the 1 pentoxide in a bomb calorimeter ; gram, of aluminium under the same conditions evolves 6970 calories.6 The calculated heat of formation of tantalum pentoxide is as follows :

2Ta+(5O 2)=Ta 2O6 +49S-3 Cals.

Tantalum pentoxide is a remarkably stable substance. It is not attacked by chlorine, hydrogen chloride, carbon tetrachloride, bromine 6 or hydrogen bromide even at high temperatures, but if it is previously mixed with carbon these reagents do attack it. When heated with phosphorus pentachloride or phosphorus trichloride in an air-free 7 sealed tube at 200 C. it is converted into the pentachloride. Sulphur is action on it without ; hydrogen sulphide produces traces of tantalum at carbon disulphide high temperatures ; disulphide produces tantalum 8 disulphide at a white heat. Carbon, at high temperatures and in the 9 absence of air, reduces the pentoxide to a carbide, TaC, or to a mixture of lower oxides 10 in the air the ; presence of a mixture of the carbide 11 and nitride is produced. Reduction with aluminium yields an aluminium-tantalum alloy. The oxide is insoluble in all acids except hydrofluoric acid. According to some investigators the solution of tantalum pentoxide in hydrofluoric acid loses some of its tantalum on 12 to content being evaporated ; according others, however, no such 13 loss takes place. It has recently been shown that pure tantalum pentoxide, prepared by oxidation of the metal, does not show any volatility when evaporated in hydrofluoric acid, and any loss that takes place has been attributed to the presence of traces of alkali in the 14 pentoxide. The amount of the loss varies with the alkali content, and the pentoxide can be completely volatilised by heating with ammonium fluoride. No loss of tantalum takes place in the presence

1 Ruff, Seiferheld, and Suda, Zeitsch. amrg. Chem. 9 1013, 82, 398. 2 Wartenberg, Broy, and Reinicke, Zeitsch. Elektrochem., 1923, 29, 251. 3 Rose, Pogg. Annalen, 1848, 74, 285 ; 1857, 100, 428 ; Marignac, Ann. Clwm. Phys., loc. cit. 1866, [iv], 9, 254; Deville and Troost, Compt. rend., 1867, 64, 294 ; Holmquist, ;

Hinrichsen and Sahlbom, Ber., 190(5, 39, 2600 ; Muthmann, Weiss, and Riedelbauch, /. Annakn, 1907, 355, 84 ; Balke, Ain&r, Chem. Soc., 1910, 32, 1131. * Friederich, Zeitsch. Physik, 1925, 31, 813. 5 Moose and Parr, J. Amer. Chem. Soc., 1924, 46, 2660 ; compare Muthmann, Weiss* and Riedelbauch, loc. cit. 6 Smith and Maas, Zeitsch. anorg. Chem., 1894, 7, 96 ; Delafontaine and Linelbarger,

J. Amer. Ghem. Soc., 1896, 18, 535 ; Ruff and Thomas, Zeitsch. anorg. Chem., 1926, 156,

213 ; Travers, Compt. rend., 1918, 166, 494. 7 Pennington, J. Amer. Qhem. /Soc., 1896, 18, 64. 8 cit. Smith and Maas, loc. ; Biltz and Kircher, JBer., 1910, 43, 1644. Friederieh and Sittig, Zeitsch. an&rg. Ohem., 1925, 144, 169. 10 Ruff, Seiferheld, and Suda, loc. cit. ; compare Slade and Higson, J. Chen. Soc., 1919, 115, 211. 11 Friederich and Sittig, Zeitsch. anorg. Chem., 1925, 143, 293. 12 64 Rose, Pogg. Annakn, 1856, 99, 481 ; Levy, Analyst, 1901, 26, ; 1915, 40, 204; 1505. Travers, loc. cit. ; Van Haagen and Smith, J. Amer. Chem. Soc., 1911, 33, 18 Ruff and Schiller, Zeitsch. anorg. Chem., 1911, 72, 329 ; compare also Schoeller and Powell, Analyst, 1928, 53, 258. 14 Hahn and Puetter, Zeitsch. anorg. Chem., 1923, 127* 153, 198 VANADIUM, NIOBIUM, AND TANTALUM. of concentrated sulphuric acid. The oxide is volatile in hydrogen 1 chloride at 900 C, When prepared by precipitation in the presence of of nitric acid or sulphuric acid the ignited material retains traces 2 these acids. Ignited tantalum pentoxide is also dissolved by fusion with potassium hydrogen sulphate, ammonium hydrogen sulphate, 3 caustic potash, or a mixture of sodium carbonate and borax. Hydrates of Tantalum Pentoxide, Colloidal Tantalum Pentoxide, Tantalic Acid. When tantalum pentachloride or pentabromide is treated with water, or when a solution of a tantalate is boiled with dilute acids, a gelatinous precipitate of more or less hydrated tantalum pentoxide is thrown down. Insoluble tantalates on fusion with potassium hydrogen sulphate and extraction of the melt with water give the gel. In dealing with double fluorides of tantalum it is necessary to remove all the hydrofluoric acid by evaporation with concentrated sulphuric acid, otherwise double fluorides are obtained, As in the case of hydrated niobium and vanadium pentoxides, it is a difficult matter to remove traces of mineral acids from the precipitate. Treatment with water gives rise to a cloudy hydrosol which passes through an ordinary filter. This can be prevented by addition to the 4 wash-water of a small quantity of ammonia or acetic acid. In the preparation of anhydrous tantalum pentoxide, traces of acids can be of removed by igniting the gel in admixture with a small quantity ammonium carbonate. Hydrated tantalum pentoxide is a white, amorphous substance. A crystalline form is stated to be obtained when tantalum pentachloride is treated with water the thrown down is dried rapidly ; precipitate 5 slowly and again treated with water. A granular variety is produced when sodium tantalate solution is treated with sulphu* dioxide and the 6 flaky precipitate is dried. Tantalum pentoxide gel becomes incandescent and loses its water content when it is heated to 500 rapidly u C.," unless it has been previously aged by washing with water. This glow phenomenon is also displayed by hydrated chromium sesquioxide, by aluminium oxide, and by titanium dioxide. The composition of the gel, when dried at 100 C., varies with the method of preparation. 7 Various hydrates have been reported, but their composition must be 8 regarded as accidental, because recent investigation has shown that continuous variation in the water content takes place with variation in the vapour-pressure. The curves obtained were very similar to those given by gels of stannic oxide and silica. A true tantalic acid is, therefore, unknown, but the name is used for the more or less hydrated pentoxide. The properties of a hydrosol of tantalum pentoxide prepared by dialysis of the aqueous extract of an alkali niobate are referred to on p. 180. Hydrated tantalum pentoxide or tantalic acid is very comparable in its properties to niobic acid (see p. 157). It is soluble in excess

1 Travers, he. cit. 2 Sears and Balke, loc. cit. 3 Weiss and Landeoker, Zeitsch. anorg. Ghem., 1909, 64, 98. * Weiss and Landecker, ibid., 1909, 64, 67 ; Ghem. News, 1909, 99, 3. 5 Rose, Pogg. Annakn, 1848, 74, 245. 6 der Abegg, Handbuch anorganischen Ghemie (Leipzig), 1907, 3, iii, 859. 7 417 Rose, Pogg. Annakn, 1857, 100, ; Rammelsberg, ibid., 1869, 136, 177, 325 ; J. Hermann, praJct. Ghem., 1857, [i], 70, 195 ; 1872, [ii], 5, 66. 8 Jander and Schulz, Zeitech. anorg. Ghem., 1925, 144, 225. COMPOUNDS OF TANTALUM. 199

1 of strong acids to form a colloidal solution. The hydrate, which is prepared by precipitation from cold dilute solutions of alkali tantalates means of dilute by sulphuric acid, dissolves in hot concentrated sulphuric and is the cooled acid, reprecipitated by diluting solution with water ; hydrated niobium pentoxide is not reprecipitated, however, under similar circumstances, and its solution remains clear.2 Hydrated tantalum differs from pentoxide titanium dioxide in that it yields a flesh-coloured insoluble addition compound with ammonium salicylate, 3 \vhilst titanium dioxide passes into solution. Whereas niobic acid and titanic acid are practically completely soluble in hydrogen peroxide, tantalic acid, when precipitated hot, is almost insoluble, and when 4 precipitated cold is only partly soluble. Certain weak acids, for example oxalic acid and tartaric acid, dissolve tantalum pentoxide, probably because of the formation of soluble heteropoly-acids.

TANTALATES. Tantalum pentoxide is insoluble in solutions of caustic alkalis and alkali carbonates, but on being fused with these substances reacts to produce the alkali tantalates. Tantalates of the metals are obtained by double decomposition, using a soluble alkali tantalate and a soluble salt of the metal. Tantalates of the alkaline earths have also been obtained by fusing tantalum pentoxide with the chloride of the alkaline earth. The tantalates display wide variations in composition, the proportions of basic oxide to acid oxide ranging from 1 : 3 to 5 : 1, but, as in the cases of the vanadates and niobates, it is probable that many of those which have been prepared and described are really isomorphous 5 mixtures of simpler compounds. According to Marignac, the only true chemical compounds are the 1 : 1 or w^a-salts, of the general formula R' the 4 : 3 or hexabasic of 2O.Ta aO5 , and salts, the general formula 6 4R' 20.3Ta 2 5 . According to a recent investigation, the members of the latter series are more correctly represented as 7 : 5 salts, for example

as : 7K 20.5Ta 2 5.24H 2O, and not 4 3 salts ; they can be alternatively of the written as derivatives hypothetical ortho-acid, H3TaO 4, in which each of the four oxygen atoms has been replaced by a co-ordinated thus for O. (TaO 4 ) group, H7 [Ta(TaO 4 ) J ; example, K7 [Ta(Ta0 4) 4].12H 2 This class of alkali tantalates is of importance because it includes the only soluble tantalates known, other alkali tantalates and the tantalates of all other metals being insoluble in water. The soluble alkali tantalates undergo ready hydrolysis ; when their aqueous solutions are boiled, precipitation of a more acid salt takes place and some alkali base passes into solution. Separation of the base in this manner is naturally favoured by the presence of acids, and even so weak an acid as carbon dioxide or hydrogen sulphide precipitates salt acids tantalic acid or an acid ; with the stronger (sulphurous acid, sulphuric acid, hydrochloric acid, nitric acid, etc., but not with hydrofluoric acid) precipitation of tantalic acid takes place readily, but excess of the strong mineral acid redissolves the precipitate. Potassium chloride

1 Jander and Sclmlz, loc. cit. 2 Weiss and Landecker, Zeitsch. anorg. Ghem,, 1909, 64, 86. 9 Lange, Zeitsch. Naturimss., Halk, 1910, 82, 29. 4 Hahn and Gilie, ZeitecL anorg. Chem., 1920, 112, 283. 5 Marignac, Ann. OUm. Phys.9 1866, [iv], 9, 249- 6 Jander and Schulz, Zeitsch. anorg. Ghem., 1925, 144, 23L 200 VANADIUM, NIOBIUM, AND TANTALUM. of and ammonium salts also precipitate tantalic acid from solutions tantalates. Arsenious acid, arsenic acid, hydrocyanic acid, tartaric acid and citric acid do not, however, hydrolyse solutions of alkali is to the formation tantalates ; this difference in behaviour attributed or of soluble salts of heteropoly-acids. Addition of caustic soda concentrated solutions of sodium salts to a solution of potassium tantalate yields a precipitate of sodium tantalate which is insoluble in the presence of a high concentration of sodium ions. Measurements of the electrical conductivity of solutions of alkali tantalates have been made, 1 but no definite conclusions are deducible as to the complexity of the ions present. Some reactions of tantalates have been described when dealing with the detection of tantalum (see p. 132). The following tantalates are known : Ammonium Tantalate, (NHJaO.STaA.SHA is obtained as a solu- flocculent precipitate by the addition of ammonium chloride to a 2 tion of 4 : 3 sodium tantalate. An ammonium potassium tantalate, also obtained the same (NH4) aO.K2O.4Ta 2O5.5H 20, has been by reaction, using potassium tantalate. the action of a Barium Tantalate, 4Ba0.3Ta 2 5 .6H 20, results from barium salt in solution on a solution" of 4 : 3 sodium or potassium tantalate. 3 Caesium Tantalates. Fusion of tantalum pentoxide with caesium carbonate and extraction with water yields monoclinic crystals of the

: addition of alcohol to its 4 3 salt, 4Cs 2O.3Ta 2O5.14H 2 ; aqueous 4 solution precipitates the 7 : 6 salt, 7Cs 20.6Ta205 .3SH 20. is Calcium Metatantalate, CaO.Ta 2O5 , obtained as a crystalline mass by fusing calcium chloride with tantalum pentoxide. Fusion of this salt with more calcium chloride gives crystals of the 2 : 1 or pyro-salt, 5 2CaO.Ta 2O5 . Calcium pyrotantalate is present in some of the natural tantalum ores, for example in fergusonite and mikrolite*

Cobalt . this Tantalate, CoO.Ta 2 5 A compound having probable composition has recently been prepared by heating the two oxides 7 together at high temperatures. Iron Tantalates. The meta-salt, FeO.Ta 2 6 , has been obtained more or less pure by fusing tantalum pentoxide and ferrous fluoride with 8 excess of potassium chloride. Iron tantalates occur naturally in niobites and tantalites. The ore tapiolite has the approximate com- FeO.Ta the position 2 5 ; tantalum may be partially substituted by niobium. 9 Lithium TLi is Tantalate, 20.5Ta 2 5 .40H 20, precipitated in six-sided plates when lithium hydroxide solution is added to a solution of 7 : 5 potassium tantalate.10 1 a Jander and Schulz, Zoc. cit. Rose, Pogg. Annakn, 1857, 102, 55. 8 kc. cit. see also Hedvall and Zeitsch. Rose, ; Zwcigbergk, anorg. Chem. 9 1919, 108, 119. 4 Balke and Smith, J. Amer. Chem. floe., 1908, 30, 1666. 5 Joly,Gompt.rend., 1875, 81, 267; AnnalesJScientifimtesdel'JiicokNormak Superieure, Paris, 1876, 125. Zeitsch. Nordenskiold, Kryst. Min., 1901, 34, 692 ; Flink, ibid., 1901, 34, 681 : Simpson, Chem. News, 1909, 99, 77. 7 Hedvall, Zeitsch. anorg. Chem., 1915, 93, 313. 8 Joly, foe. cit. 9 Dana, A New 6th System of Mineralogy (Wiley, York), 1896, ed., p, 738 ; Simpson, Vtn. 107 J. Mag., 1917, 18, ; Chem. Soc., Abs., 1917, [ii], 112, 377. 10 Jander and Schulz, loc. cit. COMPOTOIDS OF TANTALUM. 501

Magnesium Tantalates. The 4 : 3 salt, 4Mg0.3Ta 2O5 .9H 20, separates out on the addition of a soluble magnesium salt to a solution of a tantalate. Fusion of magnesium chloride and tantalum pentoxide has 1 : 1 . given large hexagonal plates of the 4 salt, 4MgO.Ta 2 5 Mercurous O.8Ta is a Tantalate, 4Hg 2 2O5 .5H 2 ? greenish-yellow compound which is precipitated by mixing a mercurous salt and a tantalate in solution. 2

Tantalates. : 3 is Potassium The 4 salt, 4K 20.3Ta 2 5,16H 2 5 prepared by fusing tantalum pentoxide with excess of caustic potash in a silver crucible the extract is in a vacuum. It ; aqueous evaporated forms monoclinic a : b : c=0-7164 : 1 : 19'. prisms, 0-5475 ; j8=95 It is stable in air and can be crystallised from its aqueous solutions 3 unchanged, but boiling the solution precipitates a more acid salt. 4 According to the more recent investigations of Jander and Schulz, the crystals obtained by the foregoing reaction are six-sided prisms of the

7 : 5 salt, 7K 20.5Ta 2 5.24H 2 ; varying proportions of tantalum pentoxide and caustic potash were employed with the same result, but evaporation was effected either over phosphorus pentoxide or on the water-bath, and the crystals were washed with alcohol. Potassium O.Ta is the 4 : 3 Metatantalate, K 2 2 5 , produced when salt is heated and the product washed with water. It is insoluble in water. 2 : is the The 3 salt, 2K 20.3Ta 2 6.6H 20, precipitated out by action of carbon dioxide on the aqueous extract of a fused mixture of 5 tantalum pentoxide and potassium carbonate. Continued boiling of this salt with water yields the 1:2 salt, K 20.2Ta 2 5 .3H 2O. The anhydrous 3 : 7 salt has also been reported. is in trans- Rubidium Tantalate, 4Rb aO.3Ta 2O5 .14H 2 ? obtained parent, colourless, monoclinic prisms by fusing tantalum pentoxide with rubidium carbonate and extracting the melt with water. It is isomorphous with the corresponding niobium salt and with 4 : 3 caesium tantalate. 6 Silver is a substance Tantalate, 4Ag 2O.8Ta 2 5 ,3H 20, yellowish-white formed by the action of a silver salt on a tantalate in solution.7 Sodium Tantalates. 4 : 3 Sodium tantalate, 4Na3O.3Ta 2 5 .25H 2O, is prepared by fusing tantalum pentoxide with caustic soda in a silver crucible excess caustic is and ; of soda removed by washing with water, 8 the residue is crystallised from hot water. Another method consists in adding a concentrated solution of sodium chloride to the aqueous extract from a fused mixture of tantalum pentoxide and potassium car- 9 bonate, when the salt separates as a dense, micro-crystalline powder. It forms six-sided plates, a : c=l : 1*0167. It loses most of its water of crystallisation between 105 and 110 C., but the remainder (apparently five molecules) appears to be water of constitution, as its removal takes place only at much higher temperatures, with deeom-

1 Joly, toe. cit. 2 Rose, toe. cit. s 259. Rose, Pogg. Annakn, 1857, zoo, 559 ; Marignac, Ann. Ohim. Phys., 1866, [iv], 9, 4 Jander and Sclmlz, loc. cit. 5 loc. Rose, cit ; Ramraelsberg, Pogg. Annakn, 1869, 136, 177, 325. 9 Balke and Smith, loc. ciL 7 Rose, Pogg. Annahn, 1857, xoi, 11 ; see also Sears and Balke, J. Amer. Chem. Soc., 1915, 37 841. 8 Rose, toe. cit. 8 Schoeller and Jahn, Anatysi, 1926, 51, 613. 202 VANADIUM, 3STEOBIUM, AJtfD TANTALUM. at 13-5 C. and in 162 of position. It is soluble in 493 parts of water parts takes with boiling water, but at the latter temperature hydrolysis place, acid. The solution is alkaline precipitation of a salt which is richer in l 2 of the fore- to litmus, According to Jander and Schulz, the composition is 7Na 0.5Ta O . gone salt when crystallised at ordinary temperatures 2 2 5 or when at 100 C. it 40H 2O Na7 [Ta(Ta0 4 ) 4].20H 2 ; crystallised O.5Ta .22H O or forms needles, the composition of which is 7Na 2 2 5 2 0. Na7 [Ta(Ta0 4 ) 4].llH 2 is obtained from the 4 : 3 salt Sodium Metatantalate, Na 2O.Ta 2 5 , the residue with water, either by heating the latter strongly and washing case or by the addition of alcohol to its aqueous solution. In the latter the metatantalate contains two molecules of water when dried at 100 C. with carbon When an aqueous solution of the 4:3 salt is treated : 3 0.3Ta O .5H or the dioxide or hydrogen sulphide, the 1 salt, Na 2 2 5 20, is obtained. 2 : 7 salt, 2Na 20.7Ta 2O6 .loH 2O,

HETEHO-TANTALATES.

The evidence for the formation of complex heteropoly-acids with of niobic tantalic acid is very comparable to that set forth in the case of tantalates are in acid (see p. 165). Solutions readily hydrolysed the addition of aqueous solution by boiling, and even more readily by acid in the mineral acids, acetic acid or succinic ; presence, however, of arsenious acid, arsenic acid, tartaric acid or citric acid no precipita- of a tion of tantalic acid takes place. Again, tincture galls yields of tantalates which have been rendered yellow precipitate with solutions acid this reaction does however, take feebly acid with sulphuric ; not, acid or citric acid. place in the presence of ordinary tartaric acid, racemic which are Tartaric acid also prevents the formation of the precipitates thrown down on the addition of potassium ferrocyanide or potassium and hinders the ferricyanide to faintly acid solutions of tantalates, in acids the precipitation of tantalic acid from solutions inorganic by action of ammonia. In all these cases it is assumed that complex acids or their salts are produced, in consequence of which the usual reaction does not take place. None of these complex acids has, however, been isolated, and only one well-defined salt, namely, potassium oxalo-tantalate> 5K 2O.Ta 2O5 . is This has been tantalic 10C 2O 3 , known. compound prepared by fusing acid with potassium carbonate, dissolving the product in water, and 3 adding oxalic acid. It undergoes hydrolysis more readily than potassium oxalo-niobate. Tantalic acid dissolves in oxalic acid, but not so freely as niobic which contains oxalo-tantalic is also acid ; the solution, presumably acid, 4 more readily hydrolysed than solutions of oxalo-niobic acid. Among naturally occurring complex tantalates the following have been observed, but exact analyses are not always available : titanotantalates, silico-tantalates, stanno-tantalates, antimonyl-tantalates, uranotantalates, urano-titanotantalates,

1 cit, a,nd Zeitech. 2315. Rose, Zoc. ; Wedekind Maas, angew. Chem., 1910, 23, 2 Jander and Schulz, loc. cit. 8 Buss, Zeitsch. anorg. Ghem., 1902, 31, 90. 4 Ckem. 26 Powell and Weiss and Landecker, ibid., 1909, 64, 70 ,- News, 1910, lor, ; Sohoeller, Analyst, 1925, 50, 485. COMPOUNPS OF TANTALUM. 203

PERTANTALIC ACID AND THE PERTANTALATES.

Pertantalic Acid, HTa0 4.#H 2O, is the analogue of perniobic acid and pervanadic acid. It differs from them in its greater stability, Pervanadic acid is a yellow substance which is rapidly decomposed on acid is a white solid being warmed ; pertantalic which can be heated for some time at 100 C. without undergoing decomposition. (This behaviour is remarkable in a per-acid.) Pertantalic acid is prepared similarly to perniobic acid, using dilute sulphuric acid and the potassium it perniobatc, K 3TaO8 .|H 20. Analysis shows that contains one atom of active oxygen for each tantalum atom, and the following constitutional formula has been assigned to it :

It is not attacked by dilute sulphuric acid in the cold, but on warming the mixture hydrogen peroxide is formed. Pervanadic acid, on the other hand, is decomposed by cold dilute sulphuric acid. Perniobic 1 acid occupies an intermediate position in the order of stability. Pertantalates are prepared by the action of excess of hydrogen : 3 alkali peroxide on solutions of the 4 tantalates ; addition of alcohol precipitates them as white, crystalline compounds which yield hydrogen peroxide with warm dilute sulphuric acid and ozonised oxygen with concentrated sulphuric acid. On being boiled with water they evolve oxygen. Their composition usually corresponds to the formula contain four atoms of active for each R* 3Ta0 4.#H 2O ; they oxygen tantalum atom, and the following constitutional formula has been assigned to them : ROCK /o

ROO->Ta< I ROO/ MX

2 The following are known in this series :

Na 3TaO8 .H 2O . . .A white, amorphous powder.

Na 3Ta08 .14H 2O . . . Pale yellow crystals. K 3TaO8 .pI 2O . . .A white, crystalline mass. Do. do. do. Rb 3TaO8 .... Cs Do. do. do. 3TaO8 ....

3 salts : .TH also the following double KCaTa08 .4jH 2O ; KMgTa08 2O ; .9H 0. NaCaTaO8 .4|H 2O ; NaMgTaO8 .8H 2 ; RbMgTaO8 2 In addition to the above, a sodium pertantalate of the composition an extract of Na2T%09 .13H 2O has been prepared by treating aqueous the residue which is obtained when a solution of 4 : 3 sodium tantalate is evaporated to dryness with a few drops of hydrogen peroxide and then adding a little alcohol. A white, amorphous powder of the composition indicated is thrown down. It yields hydrogen peroxide with : : is 1 : 1 : dilute sulphuric acid. The ratio Na 2O Ta 2O5 active oxygen 3,

1 Melikoff and Pissarjewsky, Zeitech. anorg. CJiem., 1899, 20, 344; Sieverts and Muller, ibid., 1928, 173, 297. 2 Zoc. tit. J. Chem. Soc. 1140. Melikoff and Pissarjewsky, ; BaJke, Amer. t 1905, 27, 3 Balke and Smith, ibid,, 1908, 30, 1668. 204 VANADIUM, NIOBIUM, AND TANTALTJM.

and when the salt is treated with freshly precipitated aluminium hydroxide, two-thirds of the active oxygen is retained by the precipitate, the the other third passing into the filtrate. These figures agree with constitutional formula /O /O

, 13H (X , | 2 NaO.Ta< | NaO.OTa< X \0 II |i

This sodium pertantalate, therefore, appears to be a double salt, NaTa0 4.NaO.TaO 4.13H 20. Tantalum Peroxyfluorides or Fluoroxypertantdlates, The alkali tantalum oxyfluorides also have the property of taking up oxygen by reaction with hydrogen peroxide. When potassium tantalum fluoride is dissolved in 4 per cent, hydrogen peroxide solution which contains a small amount of hydrofluoric acid, are crystals of potassium tantalum peroxyfluoride, 2KF.TaO 2F 3.H 2O, it loses water at 100 C. obtained. This is a remarkably stable substance ; and oxygen only at higher temperatures. With concentrated hydro- 1 fluoric acid it yields hydrogen peroxide. The corresponding rubidium 2 is salt, 2RbF.TaO 2F 3.H 2O, prepared similarly.

TANTALUM AND SULPHUR.

of the TaS is Only one sulphide tantalum, namely, disulphide, 2 , known, and this is prepared by a dry method. Hydrogen sulphide does not precipitate any sulphides when passed into a solution of a tantalate, it is action nor docs reduction take place ; without appreciable on 3 tantalum pentoxide even at 1200 C. Tantalum Disulphide is most conveniently prepared by passing a dry mixture of hydrogen sulphide and carbon disulphide vapour over 4 tantalum pentoxide between 900 and 1300 C. It has also been obtained by the action of carbon disulphide on tantalum pentoxide at 5 a white heat, by the action of hydrogen sulphide on tantalum penta- 6 chloride, or by gently heating tantalum in sulphur vapour, but the true composition of the products in these cases is somewhat uncertain. is Tantalum disulphide a black powder with a metallic lustre ; when heated above 1200 C. it forms yellowish crystals. It burns in air or oxygen with evolution of sulphur dioxide ; the tantalum pentoxide simultaneously produced contains sulphuric anhydride. Hydrogen attacks it does so It only feebly ; chlorine more readily. takes up traces of it retains at water, which very firmly ; high temperatures the absorbed water reacts to form hydrogen sulphide and tantalum oxides. Concentrated solutions of potassium polysulphides and boiling hydrochloric acid are action it is cold nitric without ; slowly attacked by acid, hydrofluoric acid, sulphuric acid, or a mixture of nitric acid and hydrofluoric acid nitric acid it ; boiling and aqua-regia oxidise completely to tantalum pentoxide and sulphuric acid.

1 Zettsck. /. Piccioi, anorg. Chem., 1892, 2, 24 ; Balke, Amer. CJiem. Soc., 1905, 27, 1140. 2 Balke and Smith, ibid., 1908, 30, 1666. 3 Biltz and Kircher, Ber., 1910, 43, 1636. 4 Biltz and Kircher, loc. cit. 5 Bose, Pogg. Annalen, 1856, 99, 575. * von Bolton, Zeitsch* JSlektrochem., 1906, II, 45. COMPOUNDS OF TANTALUM. 205 No of tantalum Sulphates. sulphates have been definitely prepared. A fused mass of tantalum pentoxide and potassium hydrogen sulphate after extraction with boiling water is crystalline and contains the sul- and it has phate radical, been assumed that a sulphate of tantalum is present. After drying at 100 C. the product in one case 1 had the 3Ta O .S0 .9H but this composition 2 5 3 20, was most probably the hydro- of lytic decomposition product an unstable sulphate present in the melt or in solution.2 A Cl chlorosulphatotantalum acid, H 2[Ta 3 6 .S04 ].H aO, is referred to on p. 193. TANTALUM AND NITKOGKEN. Tantalum reacts with when heated in the slowly nitrogen gas ; combination commences at about 900 C. One gram of tantalum wire absorbed 2-2 cc. of in one hour 3 nitrogen between 920 and 1030 C., and this was not in vacuo. expelled by heating In an earlier investigation 4 hours' at thirty-one exposure 1000 C. gave an absorption equal to 17*3 per cent, of the weight of tantalum. Tantalum Mononitride, TaN, has been prepared by heating the metal first in 5 hydrogen and then in nitrogen, or by the action of hy- and 6 drogen nitrogen on tantalum pentachloride vapour. It is a dark blue or black substance which melts at 3070 abs. with some loss of Its 7 nitrogen. dissociation pressures have been studied. Density =14-1. Direct heating of tantalum in nitrogen has also given rise to a black TaN is dinitride, 2 , which not attacked by concentrated hydrochloric, nitric or sulphuric acids. It evolves ammonia when fused with caustic and burns alkalis, with incandescence to the pentoxide when heated in air.8 Tritatitalum Ta has been Pentanitride, 3N6 , obtained as a bright red, amorphous powder by passing ammonia over heated tantalum pentachloride. On being heated in air it forms the pentoxide, and when fused with caustic potash it evolves ammonia.9 Nitrides of doubtful composition are also formed by the action of 10 11 ammonia or cyanogen on the heated pentoxide. Fusion of a mixture of tantalum pentoxide and carbon in excess of sodium carbonate under 12 ordinary conditions also gives rise to a tantalum-nitrogen product.

TANTALUM AND CARBON. Reduction of tantalum pentoxide with carbon in the electric furnace 13 yields carbides or alloys of variable composition, Traces of carbon 1 Hermann, J. prakt. Chem., 1857, 70, 201. 2 Compare Sears, J. Amer. Chem. Sac., 1926, 48,*346. 3 Sieverts and Bergner, Ber., 1911, 44, 2395. 4 Muthmann, Weiss, and Riedelbauch, Annakn, 1907, 355, 92. 5 Friederich and Sittig, Zeitsch. anorg. Chem., 1925, 143, 293 ; Becker and Ebert, Zefoch. Physik, 1925, 31, 271. * Van Arkel and de Zeitsch. Boer, anorg. Ohem., 1925, 148, 348 ; see also Joly, Bull. Soc. 506 Chem. 251 Mm., 1876, [ii], 25, ; New, 1876, 33, , Annaks Scientifiques de Vflcok Normal*. Superieure, Paris, 1877, 149. 7 Slade and Higson, J. Chem. Soc., 1919, 115, 216. 8 Muthmann, Weiss, and Eiedelbawch, foe. eft. foe. tit. see also Joly, ; Rose, Pogg. AnnaUn, 1857, 100, 146 ; 1859, 106, 141. 10 66 Rose, ibid., 1856, 99, ; 1844, 63, 317 ; 1846, 69, 115. 11 Rose, ibid., 1857, 100, 146. 12 Friederioh and Sittig, loc. cit. 1S Compare Ruff, Seiferheld, and Suda, Zeitsch. anorg. Chem., 1913, 82, 397. 206 VANADIUM, NIOBIUM, AND TANTALUM. render tantalum hard without affecting its ductility ; when the carbon 1 is but brittle content exceeds per cent, the product extremely hard ; tantalum containing 0-5 per cent, of carbon can, however, be drawn into wire 0*1 mm. diameter.1 Tantalum Carbide, TaC, has been prepared by heating a mixture of tantalum pentoxide and carbon in a molybdenum boat at 1250 C. 2 in a stream of hydrogen, or by the action of hydrogen and carbon 3 monoxide on tantalum pentachloride. It is a dark grey or black substance which is insoluble in all acids, and burns to the pentoxide when powdered and heated in air. Density = 13*96. It melts with decomposition at 4100 absv which is probably the highest melting- point yet recorded for a chemical compound. Its hardness coefficient 4 lies between 9 and 10. It is a good conductor of electricity. For its 5 crystal structure see reference cited.

1 von Bolton, Zeitsch. Elektrochem., 1905, n, 47. 2 Friederich and Sittig, Zeitsch. anorg. Chem., 1925, 144, 174. 3 Van Arkel and de Boer, t&t'dJ., 1925, 148, 348. * Friederich, Zeitsch. Physilc, 1925, 31, 813. 5 Becker and Ebert, ibid., 1925, 31, 269. NAME INDEX.

ABEGG, 64, 77, 87, 90, 91, 93, 198. Bergner, 177, 186, 205. Aichel, 16, 18, 134, 135, 137, 155, 173. Berkmann, 155. Airoldi, 37, 107. Berzelius, 12, 24, 00, 51, 53, 95, 104, 105, Alinari, 46, 78, 80. 107, 108, 122, 172, 180, 188-190, Allm, 175. 193. Allmand, 27. Bichowsky, 32. Amadori, 64, 69. Bikerman, 60. Anderson, 58, 112. Billy, 17. Apjohn, 1], 12. Biltz, H., 150, 169, 197, 204. Arndt, 57, 182. Biltz, W., 36, 43, 59, 150, 169, 194. Arnold, 13, 26, 105, 130. Birnbra'Tier, 174. Arsem, 19. Bjomstahl, 61. Aschenbrenner, 61. Blair, 116. Aston, 25, 137. Blake, 10. Atack, 113. Bleecker, 15, 35, 54-56, 58, 71-75, 112, 115. Atterberg, 106, 170. Bleekrode, 55. Aucliy, 57, 112. Bleyer, 17,24,45,55. AtLerbacli, E. B., 57, 115. Blinow, 102. Auerbach, F., 87, 90, 91. Blomstrand, 118, 122, 138, 142, 151, 165, Auger, 58, 92, 96, 99, 101, 177. 180. Bluli, 60. BACKA, 17, 23, 40, 41, 43, 46-48, 103, 105, Boer, de, 205, 206. 107. Bohm, 131, 155, 196. Bailey, 75, 126. Bonn, 114. Bain, 26. Bolton, von, 18, 19, 124, 134, 135, 137, Bairsto, 179. 138, 140, 142, 150, 161, 169, 170, Baker, 38, 39, 65, 66, 78, 146, 148. 172-174, 177, 183-186, 196, 204, 206. Balke, 4, 125, 129, 139, 144-146, 150, 155, Bongiovanni, 106. 159-163, 167, 168, 172, 173, 177-182, Bonrion, 43, 45, 150, 193. 189-191, 193, 194, 196-198, 201-204. Boutwell, 10, Baly, 21. Bowman, 66. Baragiola, 78, 80. Bray, 126, 127, 129, 190. Barbieri, 105-107. Brearley, 112. Bardaoh, 57, 113, Bridgman. 175. Barnes, 25, 26. Brierley, 53, 96. Barr, 124, 126, 137, 152, 153. Brinton, 72, 113, 115. Bartlett, 174. Briscoe, 25, 44. Baskerville, 12. Brizzi, 35, 42, 47, 48, 58, 107. Baughman, 17-19. Brdgger, 18, 118-121. Baumgarten, 186. Brown, 45. Baur, 135. Browning, 57, 74, 110, 115. Baxendall, 20. Broy, 18, 135, 197. Beard, 115. Brunck, 182, 196. Bechert, 20. Brttyne, de, 179. Beck, 56, 64, 70, 78, 80. Buff, 55. Becker, 103, 104, 170, 205, 206. BuHnkeimer, 125, 159. Beokman, B., 175. Bultemann, 35, 58, 97, 107, Beokman, J. W., 18. Burgess, 19, 174. Bedford, 125, 159, 162-164. Burns, 34. Behrendt, 57, 98-100. Bntzbach, 90. Benedicks, 77. Bensaude, 1L CAKREBA, 177. Benton, 56. Cain, 58, 83, 112, 116. Bezgemann, 11, 73, Camboulives, 150. Jtergholm, 61. Oampagne, 71, 112, 116, 207 208 VANADIUM, NIOBIUM, AND TANTALUM.

Canned, 10, 53-55, 65, 66, 69, 73-75, 79, Dolejsek, 21. 82, 83, 85-89, 97, 107. Domeyko, 11. Carli, de, 56, 70. Donald, 121. Carnolly, 55, 65, 66, 68-70, 74, 75, 79, 80. Donaldson, 26, Carnot, 64, 68, 72-74, 77, 115. Donati, 114. Carobbi, 10, 12, 66, 77. Doring, 10, 11. Oastendyck, 89. Dougherty, 116. Ohabrie, 72, 119, 192. Downs, 27, 34. Chakravarti, 59, 60. Dragendorff, 111. Chapin, 181, 192, 195, 196. Dullberg, 61, 63, 65, 68, 74, 79. Chapman, 35, 58. Dumanski, 59, 60. Chen, 28. Dunham. 178. Chesneau, 125. Dunn, 26. Chydenius, 196. Dushman, 177. Clark, 26, 28, 113. Dyson, 27. Clarke, F. W., 9, 138. Clarke, S. G. 106,115. EBELMEN, 155, 196. Cleve. 72, 77, 79, 80. Ebert, 103, 104, 170, 205, 206. Coblentz, 175. Eder, 137, 175. Coehn, 186. Edgar, 57, 58, 113, 116. Collet, 19. Edson, 18. Collet-Descostils, see Dcscostils. Edwards, 42, 47, 105, 106. Collie, 10, 11. Efremov, 27. Comucci, 10. Eichner, 96, 101. Conant, 40, 41, 95, 114. Ekeberg, 122. Conley, 15. Ellram, 111. Copaux, 82. Ellsworth, 118. Cork, 177. Engle, 174, 179. Cormimboeuf, 115. Ephraim, 37-40, 45, 56, 57, 64, 70, 76-78, Costachescu, 37, 39. 80, 191. Coster, 21, 137, 177. Eppley, 20. Cowper-Coles, 35. Errera, 60. Crook, 10, 120. Etheridge, 113. Crouch, 182. Evans, 106. Crow, 44, 46, 50-52, 99. Ewald, 177. Cumenge, 10. Exner, 20, 175. Curtis, 56, 57, 113. Cutter, 40, 95. FEIGE, 47, 81. Cuttica, 46, 78, 80. Feit, 192-194. Czako, 28. Fenner, 114. Czudnowitz, 24. Fenwiek, 114. Fernandez, 86. D'AGTTIAR, 15. Finger, 175. Damour, 11. Fischbach, 112. Dana, 117, 119, 120, 200. Fischer, H., 29, 37. DauvilHer, 137, 177. Fischer, S., jun., 17, 33, 35, 54, 56, 74, Davies, 177. 96, 99, 100, 115. Deering, 127. Fiath, 114. Deiss, 116. Fleck, 44. Delafontaine, 149, 151, 153, 154, 169, 197. Fleming, 182. Demarpay, 150, 193. Flight, 10. Descloizeaux, 69. Flink, 18, 200. Descostils, 12. Foa, 106. Deville, 12, 123, 124, 134, 139, 150, 151, Foote, 119, 126, 129, 130. 153, 194, 197. Forsythe, 174. Dhar, 59, 60, 78. Fourment, 16. Diefenthaler, 112, 113. Fournier, 23. Digges, 183. Frauenborger, 23, 138. Disch, 174. Fredenhagen, 178, 185. Ditte, 11, 12, 46, 50, 52, 53, 55, 56, 61, French, 183. 64-66, 68-80, 84, 86, 90, 91, 100, 102, Frenzel, 10, 11, 64. 113, 115. Freund, 127. Dittrich, 127. Freundlich, 56, 59-61. Ditz, 57, 113. Friedel, 10. Doerner, 15. Friederich, 18, 49, 51, 55, 103, 104, 154,

Dolde, 34. i 155, 170, 186, 196, 197, 205, 206, NAME INDEX. 209

Friedheim, 78-80, 84, 86, 89, 104. Hasselberg, 12, 20. Friedrieh, 31, 43, 46. Hatchett, 122. Friman, 137, 177. Hatfield, 26. Fritchle, 10. Hauer, von, 61, 76, 78, 79, 115. Fritzsche, 61. Hauser, 120, 127, 129, 157. Funk, 150, 151, 193, 194. Hautefeuille, 53, 66. Furth, 60. Headen, 118, 119, 126, 129. Hebler, 28. GAIN, 46, 48, 50, 51, 98, 99, 102-104, 107. Hecht, 88. Gall, 58. Heozko, 113. Gamgee, 23. Hedvall, 68, 72, 160, 200. Gardner, 28. Heinle, 15. Genth, 10, 11. Helouis, 13, 29. Georges, 114. Hemsalech, 20. Gerland, 14, 61, 64, 99, 100, 112. Henderson, 89. Gessner, 56, 58-60, 84. Hendricks, 21. Ghosh, 59. Henze, 12. Gibbons, 58. Hermann, 122, 123, 138, 153, 162-164, 169, Gibbs, H. D., 27. 180, 198, 205. Gibbs, 0. W., 53, 84, 86, 87, 121, 126, 165. Herrenschmidt, 29. Gibbs, R. 0., 21. Herz, 119. Giebelhausen, 29, 107. Hess, F. L., 120. Gieseler, 20. Hess, L., 56, 64. Gilbert, 58, 71, 115. Hevesy, 131, 179, 188. Giles, 124, 125, 127, 131. Hewett, 10. Gille, 127, 199. Heymann, 37-39, 191. Gillet, 26. Hibbs, 74. Gin, 14, 18, 29, 35, 107. Hidnert, 26. Giorgis, 37-39. Higson, 19, 32, 103, 104, 173, 197, 205. Glassmann, 58. Hildebrand, 124. Gmelin-Kraut, 85, 87, 88, 91, 169, 190. HiUebrand, 9-11, 13, 112, 136. Goldmann, 44, 51, 52, 105-107. Hinnuber, 29. Goldschmidt, H., 16, 134, 140, 173, 183. Hinriohsen, 173, 180, 197. GoJdsehraidt, V. M,, 154. Hirst, 182. Gonder, 150, 169. Hittorf, 16, 49. Gooch, 56-58, 71, 112, 113, 115. Hjalmar, 21, 177. Goyder, 11. Hodges, 11. Gramont, de, 2], 137, 175. Hofer, 27. Gregory, 114. Hofmann, 34, 57, 75, 120, 164. Groesbeck, 105. Holborn, 175. Grossmann, 26, 40. Holmquist, 148, 155, 159, 163, 164, 196, 197. Grotrian, 20. Holt, 43, 55, 107. Grove-Palmer, 174, 178. Holverscheit, 112. Guertler, 135, 178, 183. Honda, 19, 135. Gtdllet, 25, 26, 183. Honigschmidt, 184. Gimther-Schulze, 138, 178, 179. Horst, 49, 55, 94, 103. Gustavson, 114. Horton, 177. Guy, 177. Hostetter, 58, 83, 112. Guyard, 27, 50, 62, 76, 99, 103, 107, 108. Hothersall, 91, 112. Huber, 23, 36. HAHN, 127, 188-190, 197, 199. Hudson, 26. Hall, J. A., 64, 65, 68. Hull, 19, 173. Hall, R. D., 119, 124, 125, 128, 129, 131, Humbert, 88. 145, 147, 150, 151, 156, 165, 167, 168, Humboldt, 12. 170, 192, 193. Hundeshagen, 57. Halsing, 56. Hunter, 17, 19. Hammersohmidt, 23, 180. Huttemann, 177. Harding, 186. Hyde, 177. Harker, 177. Harkins, 177. IBBOTSON, 130. Harned, 149. Illingworth, 130. Harrison, 20. Imperial Institute Monograph, 9, 16, 28. Hart, 131, 138, 141, 157. Inglis, 178. Hartley, 20. Ireton, 175. Hartmann, 112. Itzig, 91. Haschek, 20, 175. Ivanitskii, 59.

VOL. VI. : III. H 210 VANADIUM, NIOBIUM, AOT) TAJSTTALUM.

JACK, 136. Lacroix, 120, 121. Jackson, 22. Laissus, 184. 127-129, 131, 133, Jacob, 27. Landecker, 119, 125, 198, 199, Jacobsohn, 60. 157, 159, 165, 166, 169, 196, Jahn, 125, 159, 162, 163, 201. 202. 199. Jander, 83, 198-202. Lange, H., 120, Jamcke, 81. Lange, K., 57, 115. 130. Jeffries, 173. Langley, 119, 126, 129, 136. Jelhchaninoff, 93. Laporte, Jena, 90, Larnmth, 23. 158-161, Joachims, 60. Larsson, 21, 123, 124, 134, Johannson, 111. 164. 182. John, von, 117, 125, 129, 162, 363. Lavender, 175, Johnstone, 120. Law, 35, 58. Van 60. Jolley. 175, 182. Lee, der, 22. Joly, 126, 146, 159-161, 163, 164, 170, 171, Lees, 137. 191, 192, 200, 201, 205. Leide, 129. Jones, 17, 19. Lenher, 60. Jordan, 26. Leonhardt, 56, 59, 177. Josewski, 175. Lessheim, 21, 137, 23. Just, 114. Levaditi, Levallois, 119. KAMMER, 10. Levi, 21, Kasanezky, 40. Levin, 137. A. 197. Kaufmann, 17, 44, 45, 49, 105. Levy, Q., 128, 130, 131, Kay, 94. Levv, L., 111. 157. Kaye, 177. LeWite, 127, 129, Keeney, 28, 29. Leysaht, 116. 43-45, 47. Kelley, 114. Lickfett, 37, 38, 41, 29. KelJy, 130. Liempt, van, Keunecke, 43. Liepus, 178. Kidner, 177. Liggett, 136. Kiehl, 131, 138, 141, 157. Lind, 10. 34. Kiess, 20, 136. Lindenbaum, Kimura, 119, 121. Lindh, 21. King, 20. Lindner, 192-195. 197. Kireher, 169, 197, 204. Lineba-rger, 149, 151, Kirsch, 21. Little, 25, 44. 106. Kirschfeld, 23, 36. Locke, 42, 47, 96, 101, 102, 105, KjeUberg, 14, Lockyer, 20. Klecki, yon, 111. Loomis, 10, 43, 44, 48. Knight, 120. Loring, 19, 135, 174. Knop, 155. Lovisato, 10. Knorre, 57. Ludwig, 20. Knowles, 112. LundeJl, 112. Knudson, 114. Lustig, 79, 89. Kobell, 123. Luther, 27, 34. Koenig, 112. Lutz, 41. Kohlschutter, 75, 164. Kolthoff, 114. MAAS, 127, 129, 154, 156, 184, 196, 197, Konen, 175. 202. Koppel, 17, 44, 45, 49, 51, 52, 57, 91, McAdam, 25, 26, 74, 114. 98-100, 105-107. McCabe, 114. Krause, 83, 89. McCay, 58, 112. Kremann, 42. Mache, 19. Krishnaswami, 195. Mclntosh, 18. Kropf, 114. Mack, 26. Krusch, 120. McLennan, 136. Kruss, 101, 124, 128, 142, 148, 155, 169. McWffliam, 25, 26. Kuessner, 178. Mailhe, 34. Kurowski, 67, 72. Mallard, 155, 196. Kiispert, 57. Manasse, 52, 61, 72-74, 76-80. Kyle, 12. Manchot, 29, 37, 58. Mandelin, 111. LAAK, 111. Manz, 17, 19, 23, 36, 55. Lachartre, 71, 76, Marden, 16/17, 19, 126. NAME INDEX. 211

Marignac, 123, 125, 127-129, 134, 135, 139, Neblette, 28. 141, 142, 144, 146, 153, 155-157, 161, Nelson, 34, 162, 164, 180, 183, 188-191, 197, 199, Neumann, 27. 201. Newbery, 49, 135, 153, 154, 156, 179, 180. Marino, 23, 24, 34, 35, 40, 49, 58, 95. Nichols, 156. Markowitz, E., 21. Nickell, 27. Markowitz, J., 57, 96, 97, 102, 106, 107. Niederlander, 150, 151, 193, 194. Martin, 17-19, 32, 104. Nilson, 124, 128, 142, 148, 155, 169. Martini, 111. Nishina, 177. Matignon, 16, 19, 28, 43, 45, 71, 111. Noddack, I., 118. Manner, 26. Noddack, W., 118. Mawrow, 52, 104. Norblad, 76, 78, 79. Maxted, 27. Nordenskiold, 55, 155, 196, 200. Mazzetti, 29, 43. Nordstrom, 10. Mdivani, 49, 58, 113." Norris, 26. Meaurio, 111. Noss, 42. Meggers, 20, 136. Noyea, 126, 127, 129, 190. Meimberg, 127-129, 131. Nutti, 115. Melikoff, 40, 92, 93, 166, 167, 203. OBEEHBLMAN, 112. Mellor, 114, 128. Oefele, von, 102, 120. Mercer, 179. Ohnmais, 101. Merrill, 129, 130. Olshausen, von, 135, Mertes, 43, 44. Onnes, 19. Metzener, 145. Osborne, 131. Metzger, 131. Osmond, 13, 23, 114. Meyer, A. R, 174, Osterneld, 177, 180, 182, 186. Meyer, H,, 27. Osterman, 59. Meyer, J., 17, 23, 40, 41, 43, 46-48, 57, 63, Ostwald, 33. 71, 91, 92, 96, 97, 102, 103, 105-107, Ott, 127, 146, 149, 151, 154, 155, 169. 110, 114. Owen, 135, 174. Meyer, S., 135. Michaelis, 79, 80, 84. PACIELO, 106. Miles, 27. Palmer, 74, 113, 115, 116. Miller, 120. Parisi, 50, 105, 106. Mingaye, 12. Parr, 197. Miolati, 81. Parravano, 43. Mixter, 32. Parravo, 29. Moeser, 34. Parsons, 119. Moir, 125, 131-133. Partridge, 23. Moissan, 17-19, 28, 29, 43, 55, 104, 107, Patents, 23, 27, 102, 135, 140, 172, 173, 126, 134, 135, 137, 140, 150, 161, 170, 179, 182-185. 173, 177, 185, 193, 196. Paterson, 175, 182. Mong, 97, 100, 107. Paulson, 136, Monnet, 16, 19, 28, Pawletta, 63, 91, 110, 114. Moose, 197. Pearce, 10. Morgan, 41, 44, 106. Pecheux, 174, 175. Morozewitz, 126. Penfield, 10, 11. Morris, 26. Pennington, 126, 131-133, 144-146, 150, Morsch, 175. 190, 191, 193, 197. Morugina, 175. Perrakis, 46, 49, 51. Moseley, 137, 177. Pestelli, 10, 88. Moss, 41, 44, 106. Petersen, 37-39, 105, 148. Mott, 19, 137, 175. Pettersen, 21. Mugge, 119. PMpson, 12. Mulder, 137. Piccini, 35, 37-40, 42, 47-49, 58, 95, 96, Mtiller, E., 59, llfc-114, 203. 107, 168, 204. Muller, J. H., 127. Pickard, 114. Munzing, 100. Pieck, 84, 88. Murschhauser, 53. Pied, 130. Muthmann, 16, 19, 23, 32, 35, 101, 134, Pierle, 74. 135, 137, 138, 142, 155, 156, 161, 170, Pirani, von, 135, 174, 175, 187. 173, 187, 197, 205. Pisani, 11. Myers, 130. Pissarjewsky, 63, 91, 92, 166, 167, 203. Plum, 10. NAKAZANO, 113. Pollok, 21. Naumann, 34. Porlezza, 114. 212 VANADIUM, OTOBIUM, AND TANTALUM.

Portevin, 25, 26, 183. Ruer, 137. Powell, 124-126, 128, 130-132, 166, 191, Ruff, 17-19, 31, 32, 37, 38, 41, 43-47, 104, 197, 202. 127, 128, 130, 143, 145, 150, 153-156, Prandtl, 17, 19, 23, 24, 36, 45, 53, 55, 56, 177, 180, 186-188, 190, 192-194, 196, 63, 67, 79, 88, 89. 197, 205. 202. Priestley, 22. Russ, 125, 161, 162, 165, 166, 169, 113. Pring, 49, 135, 153, 154, 156. Russell, A. S., Prior, 120. Russell, H. ST., 20. Proskurnin, 59. Rutten, 175. Puetter, 188-190, 197, Rutter, 34, 35, 95. 26. QTJIOK, SABATIEB, 34. RAPATT, 68, 70, 73, 78, 81. Safarik, 55. Ramage, 12. Sahlbom, 173, 180, 197. 14-16. Rammelsberg, 10, 11, 24, 64, 65, 69, 70, 73, Saldatwalla, 74, 76, 77, 79, 119-121, 123, 125, 169, Samuel, 21, 137, 177. 198, 201. Sanderson, 179, Ramsey, 113. Sankey, 13. Ransome, 10. Santesson, 144, 156, 161-163. Rath, vom, 10, 11. Sarker, 78. Read, 55, 105. Sato, 121. Reehou, 177. Sborgi, 138. Reed, 26, 105. Scarliarini, 37, 106, 107. Reichard, 110, 111. Scheuer, 72, 73, 79, 91, 92. Reinders, 60, 61. Schiller, 127, 128, 143, 145, 150, 177, 188, Reinicke, 18, 135, 197. 190. 193, 197. Renschler, 58. Schilling, 26, 117, 120. Renz, 58, 72, 151, 153, 156. Schirmeister, 28, 183. Restaino, 10, 66, 77. Schlundt, 10, 43, 44, 48. Reuning, 118. Schmidt, 180. Rex, von, 79. SchTnitz-Dumont, 79, 86, 104. Rhodes, 28. Schoeller, 124-128, 130-132, 159, 162, 163, Rich, 16, 17, 19, 126. 166, 191, 197, 201, 202. Richards, 174. Schoep, 10. Riedel, 59. Schramm, 107. Riedelbauch, 16, 18, 23, 32, 35, 102, 134, Schrauf, 10, 11. 135, 137, 142, 155, 156, 161, 170, 173, Schrbr, 137, 177. 187, 197, 205. Schulten, de, 66. Rinne, 55. Schultz, 198-202. Robinson, E. H., 182. Schulze, A., 174. Robinson, H. L., 12. Schuster, 61. Rodebiish, 177. Scott, E. K., 15. Rodeja, 111. Scott, W., 112. Rodolico, 83, 85, 87. Sears, 125, 181, 193, 196, 198, 201, 205. Rogers, A., 83, 87. Sefstrom, 12. Rogers, A. F., 118. Seiferheld, 153, 155, 156, 186, 196, 197, 205. Rolla, 115. Senseman, 34. Roscoe, 10-13, 17-19, 23, 24, 35, 41, 43- Setterberg, 17-19. 45, 47-51, 58, 64^-66, 68, 69, 71, 73, Shibata, 119, 121. 103, 111, 115, 121, 135, 142, 149. Shukoff, 102. Rose, H., 10, 122, 123, 125, 134, 138, 150, Siebel, 29. 152, 153, 155-157, 160, 161, 163-165, Siedener, 113, 115. 169-171, 188, 190, 194, 196-198, 200- Siegbahn, 21, 177. 202, 204, 205. Siemens, 124, 173, 182. Rose, H., 27. Sieverts, 23, 36, 177, 186, 203, 205. Rosenberg, 27. Silliman, 10, 11. Rosenheim, 63, 65, 71, 81, 83, 84, 88-91, Silverman, 10. 97, 100, 104, 107, 112, 113, 115. Simpson, 118, 119, 125, 200. Rosenthal, K, 50, Sittig, 18, 49, 51, 103, 104, 154, 170, 186, Rosenthal, 0., 67. 196, 197, 205, 206. Rothe, 116. Slade, 19, 32, 103, 104, 173, 179, 188, 197, Rothenbach, 79. 205. Rother, 177. Slavik, 114. Rowe, F. W., 184. Sloan, 41. Rowe, H. N., 177. Smith, A. W., 175. Rowland, 20. Smith, D, P., 186. NAME INDEX. 213

Smith, E. F., 12, 74, 119, 123-125, 128, UHRLAUB, 103. 129, 139, 144-147, 150, 151, 154-156, U.S. Bureau of Mines, 15, 114, 126* 159-163, 165, 167, 168, 170, 180, VALENTA, 137, 175. 181, 184, 191-193, 196, 197, 200, 203, Van Arkel, 205, 206. 204. Van Haagen, 139, 162, 163, 195-197. Smith, H. P., 14, 26. Vautin, 134, 140, 173, 183. Smith, J. L., 120, 121, 125. Vigouroux, 28. Smith, J. R., 13. Vogel, J. L. F., 16. Snyder, 45. Vogel, R., 17, 19, 29. 33, 58, 112, 133. Someya, Vogelson, 110. Sommer, 20. Voigt, 43, 150, 169, 194. 135. Sender, Volck, 70, Soudei, 26. Spanner, 177. WAIDNER, 174. 119. Spiro, 23. Walker, 14, Stabler, 35, 42, 96, 141, 149, 169. Waltenberg, 19. Stanley, 175. Walter, 21. Stenstrom, 177, Walther, 117, Stephenson, 177. Warinski, 113. Stookey, 112. Warren, 131. 197. Stoppel, 113, 115. Wartenberg, von, 18, 19, 135, 136, 175, Storey-Maskelyne, 10. Waterhouse, 126, 127, 131. Storz, 125, 152, 153, 156, 157, 194. Weber, 23, 150, 194. Strong, 21, 137, 177. Websky, 10, 11. 202. Suda, 153, 155, 156, 186, 196, 107, 205. Wedekind, 49, 55, 94, 103, 108, 127, 129, 182. Sugiura, 78. Wegelin, 50, 55, 59, 112,413, Suhrmann, 177. Weidmann, 120. Svedberg, 135, 173. Weinland, 47, 81, 119, 125, 152, 153, 156, Sweeney, 88. 157, 194. Swehten, 28. Weiss, J. M., 27, 34. Swinburne, 182. Weiss, L., 16, 18, 19, 23, 32, 102, 119, 125, 155- Szamatolski, 84. 127-129, 131, 133-135, 137, 142, 157, 159, 161, 165, 166, 169, 170, 173, TACKE, 118. 187, 196-199, 202, 205. Tammann, 17, 19, 29, 50, 56, 102, 196. Weiss, P., 19. Tanatar, 67, 72. Wells, 120. 72. Tarchi, 46, 78, 80. Welsbach, von, 110. Tartarini, 106. Wenger, 177. Taylor, 182. Wennerlof, 144. Taylor, C. E., 131. Werner, 39, 83. Taylor, W. W., 179. Wherry, 10, 21. Tchigevsky, 102. White, 103. Thalen, 20. Whitehouse, 102, Thews, 15. Whittemore, 10. 114. Thiel, 23, 180. Wiley, Thomas, 130, 150, 154, 156, 186, 187, 192- Willard, 114. 113. 194, 197. Williams, Thomson, 11. Wilms, 112. 23. Thorseus, 21. Winkler, 127-129. Thorpe, 12, 45, 55. Winzer, 120. Tiede, 174. Wirth, Wirthwein, 35, 96. Tighe, 130. 114. Tipper, 118. Witz, 13, 23, 157. Todd, 124, 131. Wohler, 12, 122, 129, 155, Tomicek, 114. Wollaston, 122. 175. Travers, 66. Worthing, 174, 114. Travers, A., 130, 197, 198. Wright, 21. Treadwell, 58, 113, 131. Wyckoff, 170. TriantaphylHdes, 104. Wyrouboff, 106, 124, 139, 150, 151, 153, 194, Troost, 123, YANG, 63, 65, 84, 90, 113. 197. 120. Trachot, 111, 114. ZAMBOITCNT, Tschermak, 11. Zealley, 118. 143. Tschernik, 119-121, Zedner, 155. Turner, 115. Zoeher, 60, 61, 200. Twyman, 137, 175. Zweigbergk, 68, 72, SUBJECT INDEX.

AESCHYJSTITE, 120. Antimonyl tantalates, 202. Alkaloids as reagents for vanadium, 28, 111. Antimony vanadium bromide, 47. Alloys, Niobium, 140. Ardenite, 89. 13. , Tantalum, 183. Asphaltites, 9, of 138. , Vanadium, 28. Atomic weight niobium, Aluminium metavanadate, 71. tantalum, 180. niobate, 159. vanadium, 24. niobium alloys, 140, 66. silicon-vanadium alloys, 28, 37. BABITJM bromovanadate, tantalum alloys, 183. hexavanadate, 76. vanadates, 71. iodovanadate, 66. vanadium alloys, 16, 17, 26, 28. metavanadate, 71. vanadyl sulphate, 100. niobate, 159. Alums, Vanadium, 97. oxalo-niobate, 166. Amine vanadates, 75. pervanadate, 92. Ammonium hexavanadate, 76. polyvanadates, 76. hypovanadous sulphate, 8, 95. pyrovanadate, 68, 115. metavanadate, 27, 49, 54, 61, 71, 115. tantalate, 200. niobates, 158. tantalum fluoride, 188, 189. niobium fluoride, 145. vanadates, 66, 68, 71, 76, 115. acid sulphate, 142, 169. vanadite, 51. oxychloride, 152. vanadium dioxyfluoride, 39. oxyfluorides, 5, 145, 146, 154. Beryllium chlororthovanadates, 67. sulphate, 8, 142, 169. metavanadate, 72. orthothiovanadate, 101. niobate, 164. oxalo-niobate, 165. polyvanadate, 76. pervanadates, 92, 93. vanadates, 72, 76. polyvanadates, 76. Bismuth orthovanadate, 64. potassium tantalate, 200. vanadyl thiocyanate, 106. pyropervanadate, 93. Bromochlorotantalum acid, 193. pyrovanadate, 67. Bromotantalum bromide, 194. pyroxyhexathiovanadate, 101. chloride, 195. sodium niobate, 164. hydroxide, 195. tantalate, 200. iodide, 195. tantalum fluorides, 180, 189. Bromovanadates, 66. oxyfluorides, 5, 191. thiovanadates, 101. CADMIUM bromovanadate, 66. titanium fluoride, 5. chlorovanadate, 66. tungsten fluoride, 5. metaniobate, 159. vanadic sulphate, 101. metavanadate, 72. vanadite, 51. niobium fluoride, 144. vanadium alum, 9597. pervanadate, 92. dioxyfluorides, 31, 39. polyvanadates, 77. vanadochromate, 102. vanadates, 77, 159. vanadous chloride, 42. vanadous fluoride, 37. fluorides, 37. vanadyl fluoride, 38. sulphate, 95-97. Caesium metavanadate, 72. thiocyaixate, 106. niobates, 159. vanadyl carbonate, 105. niobium fluoride, 144. fluorides, 38. oxychloride, 152. nitrite, 103. oxyfluoride, 144, 146. sulphates, 100. perniobate, 168. thiocyanate, 106. pertantalate, 203. vanadate, 53. tantalates, 200. zirconium fluoride, 5. tantalum fluoride, 190. 214 SUBJECT INDEX. 215

Caesium vanadous chloride, 42. Co-ordinated compounds of vanadium, 39, fluoride, 37. 40, 42, 43, 81, 96, 97, 103, 105, 106. sulphate, 97. Copper chlorovanadate, 45. Calcium bromovanadates, 66. metaniobate, 160. chlorovanadate, 66. metavanadate, 72. iodovanadate, 66. niobium fluoride, 145. metaniobate, 159. oxyfluoride, 147. metatantalate, 200. orthothiovanadate, 101. metavanadate, 72. orthovanadate, 64. -niobates, 119, 120, 159. polyvanadate, 77. pervanadate, 92. pyrovanadate, 68. polyvanadates, 77. tantalum alloys, 183. pyrotantalate, 200. tantalum fluoride, 190. pyrovanadate, 68. vanadates, 64, 71, 72, 77, - tantalates, 200. vanadium alloys, 26, 27, 29. vanadates, 68, 71, 72, 77. Cryolite, 121. vanadium dioxyfluoride, 39. Cupferron as reagent for vanadium, 111, vanadyl sulphate, 100. 115. Carnotite, 10, 11, 13, 88. Extraction of vanadium from, 15, 16. DESOHENITE, 11, 73. Cerium niobate, 164. Descloizite, 11, 68. Cerous orthovanadate, 64. Detection of niobium, 131. polyvanadate, 77. tantalum, 131. Chileite, 11. vanadium, 109. Chloroniobium acid, 143. " Dianium," 123. bromide, 149, 153. " " Didymium metavanadate, 72. chloride, 149. polyvanadate, 77. hydroxide, 149. Divanadyl chloride, 44. Chlorotantalum acid, 143, 187, 192. Double hypovanadous sulphates, 95. derivatives , 193. niobium fluorides, 144. bromide, 193. oxybromides, 153. chloride, 195. oxyehlorides, 151. Chlorovanadates, 6, 45, 66, 67. oxyfluorides, 145. Chromium-niobium alloys, 140. sulphates, 169. vanadium steels, 26. perniobates, 168. Citrovanadates, 107. pertantalates, 203, 204. Cobalt metaniobate, 159. tantalum fluorides, 188. metavanadate, 72. oxyfluorides^ 191. niobium fluoride, 144. vanadates, 11, 67, 80. vanadate, 77. poly vanadium bromides, 47. tantalate, 200. chlorides, 42, 44, tantalum steels, 183. fluorides, 37, 39. vanadates, 72, 77. oxybromides, 47. vanadium alloys, 28. oxychlorides, 44, 45. steels, 26. oxyfluorides, 38-40. vanadous fluoride, 37, vanadous sulphates, 96. vanadyl fluoride, 38. vanadyl sulphates, 99. Colloidal niobium, 135. pentoxide, 157. tantalum, 173. $Jb-tantalum, 3. pentoxide, 157, 198. Electrolysis of niobium compounds, 141, vanadium pentoxide, 58-61, 84. 149, 169. vanadous oxide, 50. tantalum compounds, 172, 196. Colour reactions of niobium salts, 132. vanadium compounds, 17, 18, 34, 49, tantalum salts, 133. 54, 58, 114. 138. vanadium salts, 32, 33, 109, 110, Electromotive behaviour of niobium, 114. tantalum, 178. Columbites,, see Niobites. vanadium, 34. Columbium, see also Niobium (117, foot- Erythronium, 12. note). Esters of orthovanadic acid, 56, 64. Compounds of vanadium, niobium, and tan- pyrovanadic acid, 68. talum, Table of, 6. Estimation of niobium, 129. Co-ordinated compounds of niobium, 144, tantalum, 129. 146, 148, 151. vanadium, 112. tantalum, 189, 191, 193, 199, Ethylamine vanadate, 75. 202. Euxenite, 120. 216 VANADIUM, NlOBItJM, AND TANTALTJM. see Vanadium di* FERGUSONITE, 120, 121, 164, 200. Hypovanadous fluoride, Ferric niobate, 160, fluoride. 49. 1 Native* 117-121. hydroxide, see Vanadium dioxide. Ferro-niobium, 140. oxide, 95. -tantalum, 130, 183. potassium sulphate, 95v -vanadium, 13, 29, 43> 46. rubidium sulphate^ 109. salts, 8, 30*33,49. , Analysis, 15, 16,

> Manufacture, 14. ferrous metaniobate, 160. 9, " l23 metatantalate* 200. Ihnenium," - 120* niobium fluoride, 145. Ilmenorutile, Indium metavanadate, 72. tantalate, 200, Intermediate oxides, 52. lodovanadates, 66. GADOLINIUM polyvaiiadate, 77. Iron 160. Gold-tantalum alloys, 183. niobates, 3. -- Native, 117-121. Group VA, General characteristics of, , niobium alloys, 140. HALEDES of niobium, 143. tantalates, 200. -- 117-121. . Constitution of, 144. , Native, 151-153. -tantalum 130, 183. 9 Double, 5, 144, 148, alloys, 73. tantalum, 187. vanadates, 71, vanadium 13, 29, 43, 46. f Constitution of, 189. aUoys, --- 16, 109. 194. , Analysis of, 15, , Double, & 189-192, --- Manufacture of, 14. vanadium, 36. , 39, of vanadium, 25. , Constitution of, Isotope 47. , f Double, 5, 37-39, 44, 164. Jleat of formation of niobium pentoxide, 156. LANTHANUM niobate, 197. -- 120, 121. tantalum pentoxide, , Native, vanadium oxides, 31. vanadate, 77. Lead 45, 66. , clilorides, 46, chlorovanadate, 81. -- see Vanadinite. Heteropolyacids, General discussion, , Native, 91. -- vanadate, Native, 11. Heteropolymolybdates, 84-88, copper 165. 102. Heteropolyniobates, 7, 142, 158, 159, dithiopyrovanadate, 73. Heteropolyphosphates, 84, 85, 91. metavanadate, 202. -- 114, 115. Heteropolyailioates, 82, 88, 89, 165, 9 Basic, 70, 73, 202. ortho 64. Eeteropolytantalates, 7, 186, 189, 200, vanadate, 92. Heteropolytungstates, 82, 85, 86, 88, 89, 91. pervanadate, 77. Heteropolyvanadates, 7, 31, 81, 91, 98. polyvanadate, 68. Heteropolyvanado-arsenates, 82, 85, 86, 91. pyrovanadate, Heteropolyvanado - arsenophosphates, 82, vanadates, 64, 68, 70, 71, 73, 77, 114, 87, 115. Hexammines of vanadates, 64. vanadite, 51. Hexammino-vanadium nitrate, 43, 103. --vanadium alloys, 17. tribromide, 47. --zinc vanadate, Native, 11. trichloride, 8, 42. Lithium metavanadate, 62, 73. -- 70. Hexaniobates, 158. , Basic, Hexatantalates, 199. niobate, 160. Hexavanadates, 76-79. orthovanadate, 64. Hexavanadic acid, 63, 79, 89. pervanadate, 92. Hydroxylamine compounds of niobic acid, polyvanadate, 62, 77. 164. pyrovanadate, 69. vanadic acid, 75. tantalate, 200. " 123 see also tantalum 190. Hyponiobie fluoride," ; fluoride, Niobium oxytrifluoride. vanadates, 62, 64, 69, 70, 73, 77. 169. Luteo vanado-phosphates, 84. Hyponiobie" sulphate, 151 see also Hyponiobium chloride," ; Niobium oxytrichloride. KAGNESIUM hexavanadate, 46, 78. Hypovanadates, see Vanadites. metavanadate, 73. Hypovanadic bromide, see Vanadium niobates, 160. tetrabromide. polyvanadates, 77. chloride, see Vanadium tetracMoride. tantalates, 201. fluoride, see Vanadium tetrafluoride. --tantalum alloys, 184. hydroxide, 31. vanadous chloride, 42. oxide, see Vanadium tetroxide. vanadyl sulphate, 100. Hypovanadous ammonium sulphate, 8, 95. Magnetites, 9. chloride, see Vanadium dichloride* Mandelin's reagent, 28, 111. SUBJECT INDEX. 217

Manganese metaniobate, 160. Niobium, Electromotive behaviour of, 138.

metavanadate, 73. , Estimation of, 129.

niobate, 161. , Colorimetric, 131. 117-121. 130. f Dative, , Gravimetric, niobium fluoride, 145. 9 Volumetric, 130. 124. polyvanadates, 78, , Extraction of, from ores, pyrovanadate, 69, 115. , Hardness of, 135. "Vanadium alloys, 28. , History of, 122.

Mercurous metavanadate, 73. , Melting-point of, 135.

niobate, 181. , Occurrence of, 8, 117-121.

tantalate, 201. , Optical properties of, 135. vanadate, 54, 115. ores, Analysis of, 118, 119, 121. 138. Mercury niobium fluoride, 145. , Passive, vanadates, 54, 71, 73, 115. , Physical properties of, 4, 135. Metaniobates, 6, 158. , Preparation of, 134. Metapervanadates, 92. , Separation of, from tantalum, 123, 124, Metatantalates, 6, 199. 128. 124. Metavanadates, 6, 53, 56, 62, 67, 70. , , tin, etc., Basic, 70. , , titanium, 124, 126.

Metavanadic acid, 61. > 1 zirconium, 126. with heat 135. , Addition compounds ammonia, , Specific of, 75. , Valency of, 3-8, 141. Addition with Niobium 169. , compounds hydro- " alum, 8, 142, xylamine, 75. Niobium blue," 142. Methylamine vanadate, 75. Niobium bromides, 152. Mikrolite, 200. bromochloride, 149. Mimetesite, 6, 24, 66. carbide, 154, 170. Mixed metal, 16, 134. chlorides, 149. Molybdenum-tantalum alloys, 184. compounds, General discussion, 3-8, vanadium alloys, 28. 141, 143, 158, 159. 87. Electrochemical behaviour Molybdo-vanadates, 83, , of, 141, vanadoarsenates, 83, 86. 149, 169. Reduction 130- vanadophosphates, 82-S4. , Pentavalent, of, 5, vanadosilicates, 88. 132, 134, 135, 141, 142, 144, 149, Monobromochlorotantalum acid, 193. 151, 153, 154, 156. 9, 11. cyanides, 170. Mottramite, " dichloride," 149. " NEPTUNIUM," 123. dioxide, 7, 153, 154. Nickel metavanadate, 74* disulphide, 169. niobate, 164. ferrocyanides, 170. niobium-tantalum alloys, 140, 184. fluorides, 143. fluoride, 145. , Double, 144, 145. orthovanadate, 64. fluorperoxy - compounds, see Peroxy- polyvanadates, 78. fluorides. tantalum alloys, 184. halides, 4, 143. vanadium alloys, 29. hydride, 142. hydroxychloride, 149. vanadous fluoride, 37. iodide, 153. vanadyl fluoride, 38. mononitride, 169. Niobates, 4, 6, 141, 158, 159. monoxide, 153. 170. , Native, 120. nitrides, 150, 169, 141. , Reduction of, 5, 130. oxides, 4, " Niobic acid," 156-158, 165. oxybromide, 152. anhydride, see Niobium pentoxide. oxychlorides, 151. Niobites, Native, 117-119, 122, 124, 160, oxyftuorides, 145, 155. 161, 200. oxyhalides, 5, 143. Niobium, Activated, 138. - oxysulphides, 156, 169. 152. alloys, 140. oxytribromide 138. Double salts of, 153. , Atomic weight of, , 137. 149, 156. , Chemical properties of, oxytrichloride, 123, 151, 139. , Colloidal, 135. , Analysis of, Double salts of, 164. , Density of, 135. 9 152, 145. , Detection of, 131. oxytrifluoride, 5, 123, 144, 133. Double salts 164. 9 Dry reactions, , of, 146, Reduction 153. 9 Wet reactions, 132. , of, 152. , Electrical resistance of, 135. pentabromide, 218 VANADIUM, NIOBIUM, AND TANTALUM.

Niobium pentachloride, 123, 149, 150, 156* Potassium fluoroxyperniobate, 168. 204. , Analysis of, 139. fluoroxypertantalate, 78. , Reduction of, 142, 149. hexavanadate, pentafluoride, 143, 155. hypovanadous sulphate, 95. 81. , Reduction of, 144. manganese vanadates, pentoxide, 4, 123, 124, 132, 141, 146, 154. metaniobate, 161. 201. , Colloidal, 156, 157. metatantalate, 74. , Hydrates of, 7, 156. metavanadate, 70. , Preparation of, 129. , Basic, 5. , Reduction of, 5, 130, 134, 135, 153, molybdenum fluoride, 154, 156, 169. nickel vanadates, 81. 162. peroxyfluorides, 7, 142, 168. niobates, 161 ? Formula 123. sesqttioxide, 5, 154. niobium fluoride, of, 132. sulphates, 142, 169. , Hydrolysis of, 145. sulphides, 168. , Preparation of, 132. tetrachloride, 149. 9 Solubility of, 128, 147. trichloride, 149. oxyfluoride, 5, 126, 128, 132, 145, 139. Niobo-oxalates, see Oxalo-niobates. , Analysis of, 154. tantalates, 165. 1 Reduction of, 146, 153, peroxyfluoride, 168. ORGANIC compounds of niobium, 130, 133, orthoniobate, 162. 142, 151, 152, 159, 166, 170. orthopervanadate, 93. tantalum, 130, 133, 152, 191, 193, orthovanadate, 65. 194, 202, 205. oxalo-niobate, 165, 169. vanadium, 30, 44, 45, 51, 56, 90, tantalate, 202. 97, 104-107, 111, 152. -vanadate, 91. Orthoniobates, 6, 158. perniobates, 166-168. Orthopervanadates, 93. pertantalate, 203. Orthotantalates, 6, 199. pervanadates, 92, 93, Orthovanadates, 6, 30, 53, 56, 62, 64. polyniobates, 161, 162. 66. 201. , Double compounds with halides, polytantalate, Oxalo-molybdovanadates, 91. polyvanadates, 63, 78, -niobates, 7, 142, 165. pyroniobate, 162. niobic acid, 130, 166. pyrovanadate, 69. tantalates, 7, 202. sodium niobate, 164. tantalic acid, 130, 202. vanadates, 80. vanadates, 82, 90. strontium vanadates, 80. Oxythiovanadates, 101. tantalates, 196, 201. tantalum fluoride, 5, 123, 126, 188, 190.

PASSIVE niobium, 138. , Analysis of, 180. 190. tantalum, 178. , Hydrolysis of, 131, 132, 190. vanadium, 23. 9 Solubility of, 128, 132, Patronite, 9, 10, 13. oxyfluoride, 130, 132, 190, 192. Pelopium, 122. tin fluoride, 5. Perniobates, 166-168. titanium fluoride, 5. Peraiobic acid, 7, 142, 166, 167. tungsten oxyfluoride, 5. Peroxyorthovanadic acid, 92. vanadates, 63, 65, 69, 70, 74, 78, 80, 81. Pertantalates, 203, 204. vanadic sulphate, 101. Pertantalic acid, 7, 186, 203. vanadicyanide, 7, 30, 105. Pervanadates, 91-93. vanadite, 51. Pervanadic acid, 7, 58, 91, 92, 166. vanadium alum, 97. Pitchblende, 121. chlorides, 42. Platinum-tantalum alloys, 184. fluorides, 37, 39, 40. vanadium alloys, 28. oxyfluorides, 5, 38-40. Polycrase, 120. sulphates, 97, 100, 101. Polyniobates, 7, 158-164. vanadoeyanide, 8, 105. Polytantalates, 7, 199-202. vanadous chlorides, 42. Polyvanadates, 6, 31, 56, 62, 75-80. fluorides, 37. 81. , Double, 80, sulphate, 97. Potassium ammonium tantalate, 200. thiocyanate, 7, 106. vanadates, 81. vanadyl fluorides, 38. antimony fluoride, 5. nitrate, 103. cadmium vanadates, 81. sulphates, 100. calcium vanadate, 80. thiocyanate, 106. cobalt vanadates, 81. vanadate, 53, 64. copper vanadate, 81. zinc vanadates, 80, 81. SUBJECT INDEX. 219

Proto-actinium, 3. Sodium niobium peroxyfluoride, 168. Psittaoinite, 11. orthovanadate, 6, 64, 65. Purpureo vanado-phosphates, 84. orthoxymonothiovanadate, 101. Pyrochlore, 119, 121, 122, 148, 159. orthoxytrithiovanadate, 101. Pyromorphite, 6, 24, 66. oxalo-niobate, 166. Pyroniobates, 6, 158. pentathiopyrovanadate, 101. Pyroperniobic acid, 168. perniobates, 168. Pyropervanadates, 92. pertantalates, 203. Pyrotantalates, 6, 200. pervanadate, 92. Pyrovanadates, 6, 53, 62, 67. potassium niobate, 164. vanadates, 80. ROSCOELITB, 9, 10, 89. pyrovanadate, 65, 68, 69. Rubidium fluoroxyperniobate, 16. stanno-vanadates, 67. fluoroxypertantalate, 204. tantalates, 201, 202. hypovanadous sulphate, 95. tantalum fluorides, 188, 191. niobates, 162. vanadates, 79. niobium fluoride, 145. vanadite, 51. oxychloride, 152. vanadium fluoride, 37. oxyfluoride, 148. oxyfluoride, 38, 40. peroxyfluoride, 168. vanadous sulphate, 97. oxalo-niobate, 166. thiooyanate, 106. perniobate, 168. vanadyl cyanide, 105. pertantalate, 203. nitrite, 103. tantalate, 201. sulphates, 100. tantalum fluoride, 191. vanadate, 53, 54. peroxyfluonde, 204. Stanno-tantalates, 202. vanadium alum, 97. Steels, Tantalum, 183.

vanadous chloride, 42. , Vanadium, 13, 25. fluoride, 37. Strontium bromovanadate, 66. sulphate, 97. iodovanadate, 66. metavanadate, 74.

SAMABIUM orthovanadato, 78. , Basic, 70. polyvanadate, 78. orthovanadate, 65. Samarskite, 120, 121, 123. pervanadate, 92. Silicides, Tantalum, 187. polyvanadates, 79. 120. , Vanadium, 107. Striiverite, Silico-molybdovanadales, 82, 88. Sulphatochlorotantalum acid, 193. niobates, 165. Sulvanite, 11, 101. -tantalates, 202. titanoniobates, 165. TANTALATES, 6, 186, 199. 89. 120. tungstovanadates, 82, -, Native, 165. Reduction 5. zircononiobates, " , of, SiHcon-tantalum alloys, 184. Tantalic acid," see Tantalum pentoxide. vanadium alloys, 16, 28, 43. Tantalites, Native, 117-119, 122, 124, 160, Silver metaniobate, 162. 172, 200. metavanadate, 74. Tantalo-niobates, 165. 202. , Basic, 70. -silicates, orthovanadate, 65. stannates, 202. 202. pervanadate, 92. titanates, 183. polyniobate, 162. Tantalum alloys, pyrovanadate, 69. as catalyst, 183. Atomic 180. tantalate, 201. , weight of, 178. vanadite, 51. , Chemical properties of, 177, 173. vanadium alloys, 29. , Colloidal, 173. Sodium ammonium niobate, 164. , Density of, Detection 13K* fluoroxyperniobate, 168. , of, 133. fluorvanadate, 66. , , Dry reactions, 132. hexaniobate, 163. , f Wet reactions, Electrical resistance 174. hexavanadate, 79. , of, Electromotive behaviour of, 178. metaniobate, 139, 163. , Estimation 129. metatantalate, 202. f of, 131. metavanadate, 25, 38, 74, 115. t 9 Colorimetric, 130. , Basic, 70. 9 9 Gravimetric, from 124. niobates, 158, 162, 163. , Extraction of, ores, 174. niobium fluoride, 145. , Hardness of, 122. oxyfluorides, 148, 164. , History of, 220 VANADIUM, OTOBIUM, AND TANTALUM.

Tantalum lamps, 124, 182. Tantalum tetrachloride, 193. 174=. , Melting-point of, tribromide, 195.

, Occurrence of, 8, 117. trichloride, 193. 175. , Optical properties of, Tapiolite, 117, 200. ores, Tables of, 118, 119, 121, Tetraethylamine vanadate, 75. , Passive, 178. Thallium hexavanadate, 80. , Physical properties of, 4, 173. metavanadate, 74. 172. 70. , Preparation of, , Basic, , Separation of, from niobium, 123, 124, niobium oxyfluoride, 148, 128. orthovanadate, 65. 79. , , tin, etc., 124. polyvanadates, _, 9 titanium, 124, 126. pyrovanadate, 69.

9 1 zirconium, 126. tantalum fluoride, 191. 39. , Specific heat of, 174. vanadium dioxyfluoride, steels, 130, 183. vanadous fluoride, 37. 97. , Uses of, 182. sulphate, 38. , Valency of, 3-8, 186. vanadyl fluoride, Tantalum bromides, 194. Thiovanadates, 101. bromochloride, 193, Thorium niobate, 164. bromo-iodide, 195. polyvanadate, 80. carbide, 197, 205, 206. pyrovanadate, 70. chlorides, 192-194. Tin-vanadium alloys, 28. compounds, Electrochemical behaviour Titanic acid, 4.

of, 172, 1783 196. Titano-niobates, 165. 165. , General, 3-8, 186. silico-niobates,

, Pentavalent, Reduction of, 131, 132, tantal&tes, 202. 186. uranotantalates, 202. dibromide, 194. uranylniobates, 165. dichloride, 192. Transition elements, 3. dioxide, 7, 196. Tungsten-tantalum alloys, 186. disulphide, 204. vanadates, 71. fluorides, 5, 188. vanadium steels, 26.

, Double, 188-191. Tungsto-vanadates, 82, 88. fiuorperoxy - compounds, see Peroxy- vanadoarsenates, 83, 86. fluorides. vanadoarsenophosphates, 87. halides, 6, 187. vanadophosphates, 82-85. hydride, 187. vanadosilicates, 89. hydroxybromide, 195. 193. hydroxychloride, 202. 196. URANO-TANTAXATBS, iodide, 202. mononitride, 205. titanptantalates, Uranyl-titanoniobates, 165. nitrides, 197, 205. vanadates, 50, 83, 88. oxides, 186, 196. oxybromide, 196. " oxychlorides, 194. VALVE-ACTION," Niobium, 137.

oxyfluorides, 188, 191. , Tantalum, 178. oxyhalides, 187. Vanadates, 6, 31, 62, 75, 80.

pentabromide, 181, 195, , Reduction of, 5, 17, 112. pentachloride, 123, 193, 194, 197. Vanadic acid, see Vanadium pentoxide. , Analysis of, 180. oxide, see Vanadium pentoxide. , Double salts of, 194. salts, 56. f Bednction of, 192. sulphates, 31, 100, 101. pentafluoride, 5, 188. Vanadieyanide, Potassium, 105. , Acid, 189. Vanadinite, 6, 10, 11, 24, 66. pentiodide, 143, 188, 196. Vanadites, 7, 31, 51. pentoxide, 123, 124, 132, 180, 186, 196, Vanadium alloys. 28. 197. , Atomic weight of, 24, , Colloidal, 157, 198. catalysts, 13, 27, 33. 197, , Hydrates of, 7, , Chemical properties of, 23. , Preparation of, 129, 180. > Density of, 19.

, Reduction of, 5, 177, 183, 186, 196, , Detection of, 109. 204-206. 197, f f Dry reactions, 111. 204. _ peroxyfluorides, 7, ? 9 Wet reactions, 109. 184. silicide, , Electrical resistance of, 19. sulphates, 205. Electromotive behaviour 34. -- , of, sulphides, 204. , Estimation of, 112. SUBJECT INDEX. 221 Estimation Vanadium, of, Colorimetric, Vanadium oxybromides, 47, 48. 114. oxychlorides, 44.

, 114. , Electrolytic, oxydichloride, 46. , , Electrometric, 114. oxyfluorides, 31, 38, 39. 114. , 9 Gravimetric, oxyhalides, 36. , , Spectroscopic, 114. oxyiodides, 48. . 9 112. 9 Volumetric, oxymonobromide, 47, 48.

, Extraction from 14. of, ores, oxymonochloride, 7, 44, 45, 49. Hardness 19. , of, oxysulphides, 95. 12. , History of, oxytribromide, 48. 19. , Melting-point of, oxytrichloride, 18, 24, 25, 31, 38, 41, 44, Occurrence 9. , of, 8, 45, 49, 50, 54, 57.

, 19. Optical properties of, , Double salts of, 45. ores, Table of, 11. 10, , Heat of formation of, 46. - 23. , Passive, oxytrifluoride, 5, 38.

, 18, Physical properties of, 4, , Double salts of, 5, 38. , Physiological action of, 22. pentafluoride, 38. 16. , Preparation of, pentasulphide, 94. , Separation of, from other metals, 110, pentoxide, 4, 31, 44, 54. 115. , Colloidal, 58, 71. heat 19. , Specific of, , Heat of formation of, 31. 13, 25. steels, , Hydrates of, 61, 63. , Uses of, 25. , Preparation of, 14, 23. , of, 4-8, 30, 112. Valency , Reduction of, 5, 16-18, 23, 24, 28, Vanadium 107. acetoselenates, 38, 39, 43, 47, 49, 50, 56, 57, 95, alums, 7, 30, 96, 97. 104, 112. 108. borate, peroxyfluorides, 7, 93. - boride, 107. selenates, 102. borotungstate, 108. selenides, 102. bromides, 46. sesquioxide, see Vanadium trioxide. 61. bronze, sesquisulphide, see Vanadium tri- 107, camphorates, sulphide. carbides, 16, 18, 23, 28, 43, 47, 104, 105. silicates, 27, 82, 88, 89, 107. carbonates, 5, 105. silicides, 107. chlorides, 40-44. subnitride, 103. chromates, 102, suboxide, 48. cinnamates, 107. subsilicide, 107. compounds, General, 3-8, 30-33, 62. sulphates, 95-101. , Electrochemical behaviour of, 17, 18, sulphides, 93, 94. 34, 49, 54, 58, 114. tetrabromide, 47. Table 33. , of, tetrachloride, 17, 18, 31, 38, 41, 43, 44. 105. cyanides, 7, , Heat of formation of, 46. dichloride, 17, 23, 40. tetrafluoride, 17, 31, 38. of , Heat formation of, 46. tetroxide, 7, 23, 30, 49, 50, 56, 57, 107. 37. difluoride, , Heat of formation of, 32. dinitride, 103. tribromide, 46. dioxide, 6, 30, 49, 56. trichloride, 30, 37, 41, 43-45. dioxyfiuoride, 39. , Double salts of, 42. salts 39. , Double of, , Heat of formation of, 46. disilicide, 107. trifluoride, 1$, 37.

ferrocyanides, 106. , Double salts of, 37. fluorides, 37, 38. tri-iodide, 48. , Double, 37, 39, 40. trioxide, 7, 30, 41, 44, 49, 56, 57. 107, fluosilicate, , Heat of formation of, 31.

halides, 4, 36. , Beduction of, 17, 18. hydride, 23, 25. trisulphide, 94. iodates, 90. uranate, 50, 83, 88. iodide, 48. Vanado-arsenates, 82, 85. mononitride, 103. arsenophosphates, 82. monosulphide, 93. cyanide, Potassium, 105. monoxide, 30. iodates, 90. nitrates, 5, 103. molybdates, 83, 87. nitrides, 23, 43, 47, 50, 102, 103. periodates, 90. nitrites, 103. phosphates, 81, 84, 85. oxides, 30, 48. phosphoarsenates, 82. , Heats of formation of, 32, 67, 83, 89. 222 VANADIUM, NIOBIUM, AND TANTALUM.

Vanado-selenous acid, 83. Vanadyl nitrate, 103. silicates, 82. nitrites, 103. stannates, 67, 91. oxalates, 107. tellnrates, 90. phosphates, 104, teUurites, 67, 89. radical, 7, 12, 32, 35, 49. tungstates, 82, 88. salicylates, 107. tungstomolybdoarsenophosphates, 87. salts, General, 7, 31, 51. vanadates, 53. selenates, 102. Vanadous acetate, 106. selenite, 102. acetyl acetonate, 106. succinates, 107. benzoyl acetonate, 106. sulphate, 27, 31, 50, 58, 98, 99. bromide, 46. , Acid, 99. 99. carbamides, 106. 9 Double salts of, chloride, see Vanadium trichloride. sulphites, 98. fluoride, see Vanadium trifluoride, , Double salts of, 98. hydroxide, 8, 37, 42, 47. vanadates, 52. iodide, 48. Volborthite, 11. oxalates, 107. oxide, see Vanadium trioxide. WdHLERITE, 120. pyrophosphate, 104. sulphate, 7, 95, 96. YTTERBIUM polyvanadates, 80. , Acid, 95, 96. Yttrium metaniobate, 120, 164. thiocyanates, 106. metatantalate, 120. vanado-arsenates, 9L niobates, 164. molybdates, 91. Yttro-ilmenite, 123. phosphates, 91. Yttrotantalite, 120-122, 164. tungstates, 91. Vanadyl acetate, 106. acetonates, 106. ZINO metaniobate, 164. arsenates, 104. metavanadate, 75. 108. niobates, 164. " borate, Vanadyl chloride," see Vanadium oxy- niobium fluoride, 145. trichloride. oxyuuoride, 5, 148. Vanadyl citrates, 107. polyvanadate, 71, 80. cyanide, 105. pyrovanadate, 70. dibromide, 38, 47, tantalum fluoride, 191. dichloride, 31, 44, 50. titanium fluoride, 5.

, Double salts of, 44. vanadium dioxyfluoride, 39. difluoride, 37, 38. vanadous fluoride, 37. , Double salts of, 38. vanadyl fluoride, 38. 101. Zirconic 4. " dithionate, acid, Vanadyl fluoride," see Vanadium oxy- Zirconium chlororthovanadates, 67. trifiuoride. niobate, 164. Vanadyl fluosilicate, 107. -tantalum alloys, 185. hypophosphite, 104. niobium alloys, 140. malonates, 107. Zircono-siliconiobates, 165.

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